by Hib Halverson
The first disc brakes used by the American automobile industry were designed in the early 1960s. Back then, state of the art, based on road racing technology from the '50s, was a multi-piston brake caliper fixed to the suspension knuckle or rear axle assembly. The brake disc or rotor bisected the caliper and had internal vents for cooling. Hydraulic pressure, generated by the driver's foot pushing on the master brake cylinder, acted on the caliper's pistons. They applied opposing forces to brake pads which squeezed the rotor and provided friction for the kinetic-to-thermal energy conversion that is braking. For 1965, Corvette was the first General Motors production car to get disc brakes.
Supplied by the Delco-Moraine Division, the new brake was a four-piston, fixed-caliper design and was used at all four wheels. The caliper was made of cast iron. Anodized, cast aluminum brake pistons float in the caliper bores. They are backed by springs that, when brake pressure is zero, take up clearance between the brake pad and disc. Sealing is provided by a U-shaped, rubber ring in a groove near the inner end of the brake piston. Its shape and orientation is such that, when the system is pressurized, its action is regenerative, ie: the more brake pressure; the tighter the seal. A secondary seal, or dust boot, has an inside edge that fits a second groove at the outer end of each piston. The boot's outer edge bonds to a steel ring that presses into the top of the piston bore. This boot is supposed to act as a contaminant barrier.
After introducing them on Corvettes, GM apparently had plans to go hog-wild with fixed-caliper brakes. Application of similar, Delco-Moraine, front disc brakes as an option on Chevrolets, Chevelles, Camaros and Novas followed two years later. However, this affair was short and sweet.
For 1969, with everything except the Corvette; GM did an about-face, going to a single-piston, floating-caliper design which, today, is the standard of the industry for passenger cars, light trucks and even medium duty trucks. Why did the fixed-caliper brake fell from grace with GM? Durability and serviceability problems were likely reasons.
With '65-'82 disc brakes, a durability issue many of us understand is corrosion. Defined as, "Gradual destruction of a metal or alloy due to chemical processes such as oxidation or the action of a chemical agent.", corrosion is caused by either: 1) contaminants in the cast iron, 2) water in the brake fluid or 3) water or other reactive material between the dust boot and the primary seal. The first results from poor metallurgy. The second results from brake fluid's affinity for water. The third comes from poor sealing.
The corrosion causes pits in the cast iron. Once the pits grow deep enough...and it doesn't take much, perhaps a few thousandths of an inch...the primary seal cannot conform to the caliper bore, then air gets in and brake fluid seeps out.
Another weakness is the secondary, "dust boot" which is only marginally effective. Abrasive material can work around the inner edge of the boot and lodge in the space between the caliper bore wall, the brake piston and both seals. As the brake pad wears, the piston slowly moves outward. The primary seal moves along the caliper bore trapping brake dust, or whatever, under the seal. Over time, this can abrade the seal enough that it begins to leak.
If the car sees use where salt is used to melt road ice, salt finds its way around the secondary seal. The caliper bore beyond the primary seal corrodes in short time due to the reactiveness of the salt. As the brake pads wear and the seals move out, they encounter the salt-induced pits and begin to leak.
It took 14 years, but Chevrolet indirectly admitted the secondary seal problem. Starting in 1979, they recommend adding beads of RTV silicone sealer in the secondary seal groove on each brake piston and, after the piston assembly has been installed, over the gap between the dust boot's outer ring and the caliper half.
During nominal brake action, brake pistons move about .010 in. which is actually the caliper spreading slightly in reaction to up to 1200 psi brake pressure. However, there's almost no movement by the primary seal's lip along the caliper bore. Piston movement is absorbed by the seal's ability to flex.
During piston travel not a result of brake application; the primary seal functions properly only if movement is limited to the seal's .010 in., "flex ability." Movement, not as a result of brake application, is almost always cyclical and usually results from movement of or irregularities on the brake disc friction surface. However, there can be rare cases where high frequency vibration (such as rapid travel over small ripples, substantial driveline imbalance use of impact power tools on suspension or brake parts) can excite the brake pistons to this cyclical movement.
If piston travel not a result of brake application exceeds the seal's .010, flex ability; the seal's lip actually moves along the caliper bore and acts as an air pump. During outward movement (see fig 1), a differential between the atmosphere and brake fluid behind the primary seal, forces air around the secondary seal, past the primary and into the brake fluid. When the seal cycles in (see fig 2), the pressure differential reverses and seal traps the air that was just sucked in. Great design...geez!
With air in the brake fluid, brake effectiveness and pedal feel are degraded. The problem is compounded when the caliper bores are pitted...then air sucks in more easily and brake fluid blows out.
Let's take one last stab at history. Why in hell did Chevrolet put the brake curse on '65-'82 Corvette owners by keeping in production a system that they knew had problems? Some possible conclusions:
1) GM sold millions of other Chevys so the tons of money it cost to retool for sliding-caliper brakes could be spread over monstrous production figures. Corvettes, only sold in the tens of thousands per year; perhaps not enough to warrant the cost of floating-caliper brakes.
2) In the late-'60s and early'70s, substantial GM resources went to exhaust emissions work. Resources necessary to redo the brakes probably would not have been available, even if money had been.
3) Fixed-caliper brakes, judged only by their suitability for racing, have distinct advantages. It's no secret that Zora Duntov saw Corvette as, first and foremost, a high-performance sports car and his crusade was to accept few compromisesâ€“perhaps even at the expense of durability and service ease. There is speculation that Zora would have opposed floating-caliper brakes.
4) After Duntov retired, why didn't Dave McLellan push an upgrade? In the 1975-1980 period, the obstacles of 1 and 2 above still existed and, perhaps, were more formidable. After 1980, everyone was focused on the "new" car, which would arrive in 1984 with, what elseâ€“floating-caliper brakes.
Enough theory and finger pointing! Let's look at fixes.
If you don't work on your car, understand that servicing '65-'82 brakes can be a challenge. Skill, good equipment and knowledge are required. Some Corvette "experts" are expert at only average brake jobs on these cars. This is because, since fixed caliper brakes went out of widespread use 25-years ago, the experience base in the mainstream service trades today supports floating-caliper brakes which require less servicing skill. It's buyer beware and your responsibility to ensure those doing the brakes on your Corvette respond to the system's idiosyncrasies.
If you do your own work, this article is not really " a basic repair" reference. For that, see a series on brake jobs in the June, and July, 1991 issues of Vette Magazine, a piece on front wheel bearings in the October, '93 issue of Vette and an article on brake noise in the December, '93 Vette . Other good sources are the Corvette Service Manual for your model year, which is available from Eckler's, along with the'65-'82 Brake Installation and Service Manual published by the Stainless Steel Brakes Corporation.
What you get here and in Big Block from Hell Part 12, are solutions to the some of this system's peculiar problems that have not been discussed widely. Some of this came out of a brake overhaul Global West Chassis Components did for us on our project car. In the process of engineering new products; Global's Director of Engineering, Doug Norrdin, has worked with the C2/3 Corvette chassis for several years, so we retained them for the work.
Brake disc run out, brake disc thickness variation or "non-parallelism," disc-to-axle flange inaccuracies and play in wheel bearings are all root causes of air pumping.
Run-out exists when a given point on the brake disc does not rotate in a constant plane, ie: that given point runs in-and-out or wobbles. This can be caused by inaccuracies in the rotor itself or, at the rear, by the rotor and rear axle flange surfaces not being parallel. If a brake disc has a thickness variation, its inner and outer friction surfaces are not parallel.
Both of these situations, run-out and nonparallelism, cause the brake pistons to move in and out as high and low spots on the brake disc pass between the brake pads. This produces the cyclical, non-brake pressure induced piston movement that gets you air pumping.
The most accurate measurement of brake disc run out and nonparallelism comes when the rotor in question is installed on a brake lathe, a piece of shop equipment used to machine rotors prior to use. A dial indicator is fixed to a magnetic base mounted somewhere on the lathe. The indicator stem is adjusted such that it is perpendicular to the rotor friction surface and touches the surface near the rotor's outer rim. Both friction surfaces should be measured. If the quantities are different, not only is there run out, but nonparallelism as well, a figure that is derived by subtracting the two run out numbers. In the front, precise measurement of run out can be made on the car as long as the wheel bearing clearance is temporarily reduced to zero by tightening the bearing adjuster nut to 15 ft/lbs. Run out of rear brake rotors can not be measured accurately with the rotors on the car because the effects of rear wheel bearing clearance cannot be eliminated. Rear rotors must be removed and chucked into a brake lathe.
Now, consider wheel bearing clearance. It causes axial movement of the brake disc which can affect the brake pistons the same way as run-out. Obviously, bearings must have some clearance. However, the more clearance that exists; the less rotor run out is acceptable, because the effects of both are cumulative, but yet, cannot exceed the .010-in. limit, otherwiseâ€“you suck air.
Proper procedure must also be followed to correctly measure front wheel bearing end play or clearance. One good way of doing this was discussed in Vette's October '93 story on front wheel bearings. A more accurate, but more complex alternative is to use Global's method. A special tool must be made consisting of a piece of flat, steel stock welded across the top of a wheel bearing adjuster nut. This tool is threaded onto the spindle and hand tightened against the adjuster nut. Again, we use a dial indicator on a magnetic base. The base is attached to the section of flat stock such that the indicator stem is parallel to the spindle axis and can touch the rim of the hub just outside of the wheel bearing. Slip a big screw driver into one of the rotor vents. Push on the rotor and watch the indicator.
Doug Norrdin prefers this method of checking bearing clearance because it minimizes the multiplicative effect of the brake disc's radius which can cause significant error in the reading. To test this, we set one of the '71s wheel bearings to .0005 end play at the hub, reset the dial indicator, then measured .003 at the rim of a rotor we knew had zero run out. The proper method of measuring rear bearing clearance is similar and is discussed later in this article.
As he disassembled the brake and wheel bearing systems on the BBfH, Doug Norrdin measured run-out and clearance from various sources.
LF RF LR RR
general run-out .0038 .0045 .010 .005
rotor run-out .0028 .0035 .006 .009
axle flange run-out n/a n/a .0025 .002
rotor nonparallel .000 .000 .0005 .000
bearing clearance .001 .001 .001 .001
In the one of the rear columns, the numbers don't add up because, if the factory installed rivets have been removed, the rear brake discs come off the axles and can be installed in one of five orientations. On the right, the disc had been installed such that the high points of the axle flange and brake disc were not in line. Good thing because, if it had been, we'd have been way over the .010 in. limit. We should note, that if both the flange and the rotor have run out and the rotor does not have a problem with nonparallelism; you can sometimes solve a run out problem simply by changing that orientation.
There is no flange measurement for the front, because the hub rivets to the brake disc and, if there's hub flange run-out, it "disappears" when this assembly is machined before use.
A brake disc that runs-out excessively must be machined to eliminate the problem. Also called "turning" or "truing" is accomplished with a brake lathe. The rotor is installed onto the lathe spindle. Two cutting tools are positioned such that they can machine the two friction surfaces perpendicular to the disc's axis and parallel to each other.
The controversy is how much run-out warrants turning a rotor. Chevrolet specifies .005 in. over the wheel bearing clearance as the figure beyond which you'd do a session on the brake lathe. If we accept .010 in. as our maximum, total run-out figure; obviously, Chevy's limit for rotors is barely acceptable because, if you have front wheel bearings at their Chevy limit (.008), with maximum rotor run-out; you'd be over .010.
Where the factory rotor run-out limit really falls short is at the rear because of the cumulative effect rotor run-out, rear bearing clearance and axle flange inaccuracies. The solution? For all rotors, adopt .003 as your brake disc run-out limit and .0005 your nonparallel limit.
There are times you don't turn rotors. Yep, the idea that brake discs need to be machined every time you do a brake job is a myth propagated by manufacturers of brake lathes and rotors. Our recommendation: if run-out is .003 or less, if non-parallelism is .0005 or less, if rotor thickness averages 1.215 in. or more and scoring of the friction surfaces does not exceed a depth of .015 in.; don't turn the rotor.
Conversely, when you buy a new rotor whose manufacturer says it can be used with out turning; don't believe it. Check all new rotors for run out. We ordered a set of Vette Brakes & Products heavy-duty front brake discs (p/n 13752) for the Big-Block from Hell. We were told they can be used right out of the box, but both exceeded our run-out limits at .006 and .008, respectively. However, don't think for a minute that this is a problem unique to aftermarket products! We also ordered two stock Chevy brake rotors (p/n 3996663) for the rear of the car. They, also, were way out of spec. (.006 run out, .002 nonparallelism for one and, .002 run out, .001 nonparallelism for the other) and, obviously, had to be machined. At the controls of Global's Ammco Brake Lathe, Doug Norrdin did all four rotors. Run-out, measured on the brake lathe, of the finished product was consistently 0-.0005. The bottom line, here? Check all new rotors for run-out.
A finishing touch was to lightly abrade the friction surfaces of all the brake rotors with the sanding discs in a Standard Abrasives Motorsports Division Disc Brake Prep Kit (p/n 260002). This produces a non-directional scratch pattern after rotor machining. It helps to bed-in brake pads and in some cases (mainly when using production type pads) aids in eliminating brake noise.
An electric drill or, better yet, a die grinder having a 1/4 in. chuck can be used to spin the sanding disc. The trick was to do this is while the brake disc is still spinning on the brake lathe. However, it can also be done with the rotor flat on a table.
Basic service of front wheel bearings has been covered in numerous magazine articles and the factory Service Manuals. However, we offer additional advice relating to how front bearings can be adjusted to get minimum clearance, an advantage in the battle against cumulative brake disc run out.
We said that allowable front wheel bearing clearance is .001-.010-in. with .003-.005-in. as preferable. Doug Norrdin commented, "No doubt, setting wheel bearings to a specific clearance is an acceptable method, though I disagree with your maximum spec. of ten thousandths.
"If you have that, you're already at what is generally understood to be the limit of the seal's flexing ability. Also, you have no leeway for rotor run out elsewhere. It's not uncommon to see 2-3 thousandths develop with normal use.
"If your bearings are on the loose side of that range and you add 2-3 thousandths run out; air pumping will show up. My suggestion is that you adjust the bearings for a specific torque figure."
It is quite interesting how both Chevrolet's front bearing adjustment procedure and clearance specifications have changed over the years. In 1963, when the design was introduced, the Service Manual had us torque the bearing nut to 12 ft/lbs., back off one nut flat and install the cotter pin. As a secondary check, 0-.007 was stated as a clearance range. In the 1971 Manual, the checking procedure was the same but the clearance range changed slightly to .001-.008. Apparently telling people it was okay to set tapered bearings at zero clearance cost Chevrolet some warrantyâ€“but, then, tapered bearings were new stuff in '63.
In shop manuals for the last model years of production of that platform, they had us still torquing the nut to 12 ft/lbs, but then backing off the nut until it's "just loose" (amazing they'd say thatâ€“what is the definition of "just loose?"!). Finally we're instructed to hand tighten the nut until one of the cotter pin holes in the spindle lines up with the slots in the nut. Additionally, Chevy must have gotten hip to the cumulative run-out/clearance problem as the specification for checking went down to .001-.005.
The factory would never have had to specify such a wide clearance range if they'd had the tightening procedure right to begin with. Screw whatever tradition there is in wheel bearing service and let's get the bearing clearance right so we can get the brakes to work reliably, okay folks!
During disassembly of our '71's brakes, all four front wheel bearings were inspected. Three were in excellent condition, but the right, inner showed wear. We cleaned the three good wheel bearings, ordered up a new Timkin inner (p/n LM48548) from General Bearings Co. in Los Angeles, then packed all four with Red Line CV-2 Synthetic Grease. The Vette Products rotors come with bearing races already installed, so Norrdin put the inner bearings and grease seals in the hubs, slid the brake rotors in place, installed the outer bearings, added the washer and nut, then got out his torque wrench.
The adjustment procedure used by Global West was and the procedure we recommend to you is: 1) while spinning the rotor in the direction of forward vehicle travel, torque the bearing nut to 15 ft/lbs. This sets the bearing, the grease and the races, 2) back off the nut until it's loose, 3) again, while rotating the brake disc, re-tighten the nut to 50-60 inch/pounds. In most cases, this sets clearance to about .0005-.001 in., the figure we measured after front wheel bearing service.
To do this job right, you need to buy or borrow an inch/pound torque wrench. Excellent examples are two units made by Mac Tools (p/n TWDM75IN or TWDX150IN). A less desirable method is to convert the adjustment to 5 ft./lbs then use a more "typical" torque wrench you are liable to already have available for head bolts and such. If you choose the latter, do so with care. With a torque wrench that large, the final measurement will be at the low end of the scale close to the tool's limit of measurement.
What do you do when you've got the right torque, but can't line up the nut to install the cotter pin? Forget standard practice of backing off to the first available cotter pin hole...that screws up our wheel bearing adjustment. The solution is a valve spring shim. They come in a variety of diameters and in thicknesses of .015- and .030-in. The correct thickness to use can be determined by trial and error. If you can't line up the cotter pin; remove the nut, place a shim between the nut and the wheel bearing washer, then repeat the adjustment procedure.
The rear wheel bearings on '65-'82 Corvettes present more of a service problem because of their substantial interference fit to the axles and the use of specific shims rather than an adjusting nut to set clearance. When we say "substantial" press fit, we mean it. I've watched technicians try to disassemble the rear bearing stack using a hydraulic press capable of 20 tons pressure. I once even saw a guy, pushed to desperation, break the cage off the rollers, then remove the inner bearing race with a cutting torchâ€“yes, my friends, the flaming hack sawâ€“and he did it without damaging the axle.
Whether you are successful in removing the old bearings or need to buy new axles from Chevrolet (p/n 3872476); we suggest reducing the diameter of the bearing locations such that the 20-ton press fit becomes a tight slip fit requiring only stout smack with dead-blow hammer to install the bearing and a small press to remove them. The amount material you remove is about .0003 in., (that's three ten-thousandths of an inch). This is best done with a strip of medium grit sandpaper. Standard Abrasives Motorsports, 240-grit, 2 in. wide Handy Rolls work great except for their 50-yard lengths. Hopefully you can find a retailer who will sell you a strip a couple of feet long for your axle job. Spin the axle with the lathe. Hold the strip tightly at both ends then apply the center to the axle.
However, do not just cut the axles down .0003 in. and put the bearing stack together. Our measurement is only a starting point. Cut the axles to near our figure then try on the bearings. If the fit is still tight; "trial-and-error it" until you get it right. Use caution, if you cut too much; you buy more axles. You will need a vernier or digital micrometer to make the measurements.
After the shaft diameter reduction; check the flanges as they, also, have a part in the cumulative run-out problem. We've seen as much as .003 in. on brand new partsâ€“all it takes is some errant, warehouse wonk dropping a box containing an axle. Used pieces? Doug Norrdin told us they can be worse, as much as (gulp) .010 in. run-out!
There is no factory spec for axle flanges, but we feel maximum acceptable is .002 with 0-.0005 desirable. To measure this properly, the axle should be indexed in a lathe, then the flange run-out measured with a dial indicator. In our case, the two new axles we ordered from Chevrolet each were perfect...zero run out.
If you find run-out more than .002; remove the wheel studs, then have a machinist true the outer surface of the axle flange. If the axles are new, reinstall the studs; if you are reusing your old axles; install new studs.
Chevrolet cites a rear wheel bearing clearance range of .001-.008 in. and further states that, if the clearance is within those specs; adjustment is unnecessary. Not. That's a really dumb recommendation. If you have bearings near the factory-accepted maximum, your brake disc run out is .004 and your axle flange is off by, say, .003 (.007+.004+.003 equals, ahâ€“.014); you'd have an air pumping problem. That large range makes us wonder if, back in the old days, the folks writing these specs were aware of air pumping and if they ever drove or fixed Corvettes.
The ideal situation is to set the rear bearings at .0005-.001. However, since Chevrolet only makes the shims available in .006 in. increments (again, people making decisions long ago didn't drive or fix these carsâ€“thank god they're retired), there's only a slim chance that you'll hit the magic number. The solution? Adjust the thickness of your rear bearing shims such that they become "specific-fit" devices and rear bearing clearance ends up .001-in.
Use the existing shim as a base line. Assemble the bearing stack, but leave out the inner and outer seals and do not pack the bearings. Add the inner u-joint flange and tighten the nut.
As you tighten the nut, you must constantly feel for bearing tightness by rotating the carrier as you pull down the nut. The reason for this is, if your shim is too small, the bearings will reach zero clearance before the nut reaches its suggested torque figure of 100 ft/lbs. It that happens; you can destroy the bearings. If the bearing tightens up before you reach 100 ft/lbs. Stop! Find out what's wrong, first.
If all is well, once you've got the nut at 100 ft/lbs, measure the clearance. From that number, subtract .001-in. then add the result to your existing shim. If that sum is the same as an available shim (1-in-6 chance, you lucky dog, you!), install that shim and recheck the clearance. Example:
.007 measured clearance .127 existing shim (p/n 3820228)
-.001 desired clearance -.006 clearance reduction
.006 clearance reduction .121 desired shim (p/n 3820229)
If the sum is not the same as an available shim, pick the shim of the next larger thickness above your computed, desired shim and use a surface grinder to reduce the thickness of that unit to the thickness called for by your computations. Examples:
#1 .008 measured clearance .127 existing shim (p/n 3820228)
-.001 desired clearance -.007 clearance reduction
.007 clearance reduction .120 desired shim
#2 .121 next larger shim (p/n 3820229)
-.001 surface grind
.120 desired shim as in example #1 above
Check the clearance. If it's correct, disassemble the stack, pack the bearings, install the seals and reassemble all this per the Service Manual. We recommend Loctite 242 to lock the bearings to the axles.
This trick works well in limiting bearing clearance and is very durable. In the case of the Big-Block from Hell, the rear bearing clearance had been set by its owner, using the above method, to .001, ten years and 20,000 miles previously and the chart we presented earlier show that the clearance held throughout that period.
All that was necessary during the work Global West did on this car, was to knock the Timkin bearings off the axles, clean and inspect them, repack the bearings, install new oil seals, Loctite the bearings and reassemble the bearing stack.
We began this article with a lengthy discussion of the corrosion problem generic to production '65-'82 brake calipers. The solution to rotten brakes is simple: install calipers having piston bores fitted with stainless steel sleeves. No secret there, eh...the rebuild industry developed this process in the mid-1970s and it's become the accepted fix for corroded brakes.
If a car has non-sleeved calipers, even units made up with new, Chevrolet caliper halves; corrosion will eventually begin. Once it starts, it cannot be reversed, no matter how many times you hone the caliper bores. Watch our lips...stainless-steel-sleeved calipers are required for acceptable brake durability on any '65-'82 Corvette.
Sleeving calipers is not something the do-it-yourselfer or even a repair shop can do. Specialized equipment is needed to perform machining operations specific to the process. Fortunately, there are many rebuilders that recondition Corvette brake calipers with stainless steel sleeves. For BBfH, we chose Vette Brakes & Products of St. Petersburg, Florida. Their calipers are available singly, in sets of four, or in "Master Set" form which adds brake pads, brake hoses, rear caliper brake lines, a master cylinder and brake fluid...a hell of a deal at 400 bucks!
Vette Brakes & Products' sleeves are cut from surgical-grade, stainless steel tubing. After installation, their bores are finished with a proprietary process that leaves a higher quality surface than did GM, originally. VB then installs new aluminum pistons, stainless piston springs and stainless fittings. The calipers carry a 90-day, unlimited warranty and a lifetime, limited warranty against leakage.
Earlier we discussed at length the substandard sealing system used on the '65-'82 brake pistons. In the last few years of that platform's production, Chevrolet became aware of this problem and in factory Service Manuals for the â€˜79-'82 model years, recommend a modification using silicone RTV sealer.
We disassembled each caliper, carefully pried out its dust boots, removed the piston assemblies and performed this field fix. A bead of Valco Cincinnati Silicone Sealer was applied to the groove into which the outer piston seal or "dust boot" seats and, once the piston assembly has been installed into the caliper half, you apply a bead of RTV to the parting line between the cast iron of the caliper and the dust boot. Valco Silicone comes in several colors and we used silver because it matched the silver coating on the Vette Brake and Products calipers.
Then, we reassembled the calipers. Use a torque wrench when tightening brake caliper cross bolts. Bolts on front calipers must be tightened to 130 ft-lbs. and bolts on rears to 60 ft-lbs. Many books (including some factory manuals) have incomplete torque specifications for the cross boltsâ€“sometimes only the front number is listed. Inadvertently tightening the rear cross bolts to the front figure will damage the calipers.
Even if you do not disassemble calipers to RTV the pistons; we suggest checking cross bolt torque. Many rebuilders use non-torque sensing, impact wrenches to tighten the bolts. They will tell you they adjust the impact such that it tightens the bolt to a specific torque, but that method of controlling bolt tightness is inaccurate. Over the years, we have checked many rebuilt calipers and found bolts as much as 50% under the recommended specification.
Incorrectly tightened cross bolts pose a substantial safety problem. The caliper flexing that results from normal brake use will cause undertightened bolts to loosen. Eventually, the caliper halves separate and the o-ring sealing the brake fluid passage running across the caliper blows out causing a massive fluid leak. This is most likely to happen under maximum braking, so a failure of this nature would be catastrophic causing loss of control of the vehicle. Regardless of the source of the brake calipers; check the torque of all cross bolts.
You may run across calipers already fitted with stainless steel sleeves that simply need new seals. Since stainless sleeves don't corrode and hold their surface finish better than cast iron, a sleeved caliper will often need only minor cleaning to be reused. Do not use any oil-base solvent to clean brake parts. Use brake fluid or a product intended for cleaning brake parts, such as Permatex Brake and Parts Cleaner. For stubborn deposits, Vette Brakes & Products tells us that sleeved caliper bores can be wiped out with a very fine-grained abrasive pad, such as Standard Abrasives Motorsports Division's Brite Rite, Ultra Fine, Hand Pads (p/n 827500)
The next step is to clean the pistons and install new seals. VB offers caliper rebuild kits that include the primary seal, the dust boot and O-rings for sealing the cross-caliper passages. They are available per caliper (p/n 11301, front; 11302, rear) or for the complete car (p/n 11305). Do yourself a favor and add VB stainless piston springs (p/n 11611, front; 11612, rear). When you install the primary seals, coat them with a brake parts assembly lubricant. When you install the dust boots, use the RTV silicon technique discussed elsewhere in this article.
You have four choices in brake pads: 1) production (organic), 2) street high-performance or autocross (mass marketed semi-metallic), 3) aggressive street or mild racing (soft carbon/Kevlar or aggressive semi-met.) or 4) all-out competition (aggressive carbon/Kevlar or full metallic).
Many '65-'82 drivers are probably not exploring the limits of their classic Corvette on a regular basis. If that's you; stock brake pads are great. They can be had from Chevrolet (p/n 5452515), aftermarket suppliers like Vette Brakes & Products (p/n 11401) or discount stores (Trak Auto, Auto Zone, etc.). If you run hard once in a while or do a little autocrossing; upgrade to a semi-metallic. These can be had through VB (p/n 11402). If you're a serious canyon racer or do road racing events; you need a specialty brake pad. BBfH part 13 is an article on high-performance and racing brake modifications. See that for additional information.
While installing the the brake pads, Doug Norrdin, reminded us to inspect the caliper cross pins. If they show substantial galling, are rusted or otherwise worse for wear; replace them. Screwed up cross pins inhibit smooth brake action. We prefer stainless steel units from VB (p/n 11636) for their anticorrosive properties.
Install the front calipers per the service manual. Use a torque wrench to tighten the the caliper mounting bolts to 70 ft-lbs. As for the rear, we've had the rear wheel bearings apart to re-set their clearance, so our installation began from scratch.
Global's Doug Norrdin installed the caliper mounting brackets and the e-brake assemblies to the rear trailing arms. The '65-'82 Corvettes use six-inch, drum-type emergency brakes. Drum brakes rely on moving parts exposed to moisture and road dirt. On a car with drums as all or part of the service brakes, this is not a problem. Constant use keeps pivots, levers, springs and adjuster wheels freed-up.
With the Corvette e-brake, less than frequent use has corrosion posing a problem that increases wear, stretches the e-brake cables and increases application effort. Once the adjuster screw corrodes, if not periodically operated; it eventually rusts solid. This is a common problem due to ignorance of the two-step, e-brake adjustment required on these cars. A frozen adjuster may prohibit removing the rear brake discs because you can't retract the e-brake shoes. First, back-off the secondary, cable adjustment. Then, try spraying a penetrant, such as "Aerokroil", onto the adjuster through the hole in the rotor. After a while, you should be able to turn the wheel. If that doesn't work, use a cold chisel. A few stout blows should free the adjuster.
We have two solutions to prevent any of that: 1) a kit of anti-corrosive, stainless-steel E-brake pieces from Vette Brakes & Products (p/n 11665) and 2) adjustment of the e-brake assembly per the Service Manual.
Once the e-brake units were in place, Global's next operation was to add to each arm the bearing carrier/spindle assemblies we'd built-up. Just before you install each arm is a good time to inspect the universal joints at both ends of each axle drive shaft. Global West found one that needed replacement and installed a new, Chevrolet u-joint (p/n 7834387). A completed trailing arm was installed on each side followed by the brake discs and calipers. Torque the rear caliper mounting bolts to 70 ft-lbs.
Once all the calipers are mounted, check the position of each over its rotor. Sometimes they'll be off-center to the inside or outside and sometimes angled right or left. This is often due to GM's sloppy machining of the caliper mounting bosses, but also can result from service operations that change the position of the brake disc in relation to the caliper. While a caliper slightly off-center is okay, angling will cause uneven brake pad wear. However, in extreme cases of either problem, installation of new brake pads may be prevented or, if the pads are installed with the caliper, the brakes may drag or even lock once the caliper mounting bolts are tight.
To varying degrees, we had these problems on BBfH. All calipers were off-center and one of the rears was angled. Global West measured angled unit and found a .010-inch difference in the thickness of the caliper mounting bosses.
If a rear caliper is off-center or angled to the outside or a front caliper of is off-center or angled to the inside, simply remove it and add flat washers between the caliper and caliper mounting bracket such that the problem is eliminated. Special washers, in .015-, .020-. .025 and .030-inch thicknesses, are made by Global West to solve caliper position problems.
If off-center or angling of a rear or front caliper is opposite to that listed above, you have two choices: you can fix the angling with Global's shims then, if the remaining off center is small enough to allow the brake pads to fit and not drag; live with the problem or...you can measure the off-center and angling then fix it by milling the caliper mounting bosses a corresponding amount.
Once the brake calipers were correctly positioned, Global West installed stainless steel trailing arm brake lines (p/n 11654 left, 11653 right) from Vette Brakes & Products. We also inspected all of the car's rubber brake hoses. Rubber brake hoses are the weak link in most brake systems. The rules-of-thumb for replacement are: do it if a) the rubber outer sheath is cracked anywhere b) the hose is bulged or c) it's been more than 5 years or 50,000 miles since the hoses were replaced. Lastly, the place brake hoses often fail is right where the fitting is crimped. Check that area closely. The BBfH's hoses all needed replacement, so we installed a set of new units from Vette Brakes (p/n 11733) along with a set of new c-clips and washers (p/n 11734).
After our brake line and hose work, the final step, at least at the wheels, was to reconnect the axle drive shafts. Then we moved on to some other items.
Are you working on a car with power brakes? Most folks ignore the brake booster during a brake job. A brake booster relies on engine vacuum acting a large diaphragm to amplify brake pedal input. Between 1965 and 1982, several different boosters were used. Generally, they contained two diaphragms along with numerous additional rubber and plastic parts. You'd think boosters either work or they don't, right? Wrong.
Even the newest of factory units is now 13 years old, so it's likely any booster, known to be original, contains rubber and plastic parts that, regardless of mileage, have deteriorated. The unit might function, but it probably isn't working right. Pedal travel and pedal effort may not be correct or the booster may be leaking. Over time, the driver may never notice that the quality of operation has degraded. We checked 1965, 1971 and 1982 editions of Corvette Service Manuals and found nothing on booster performance checks. While we did find information on how to overhaul power boosters; the task requires special tools that few, if any, hobbyists or even repair shops will have.
Our suggestions: 1) if the booster has failed; obviously, replace it, and 2) if the booster "feels" OK, but is more than 10 years old, you'll probably see an improvement in brake action and feel if you replace it. For '67-'82, try a booster/master cylinder combination rebuilt by Wagner and sold by Vette Brakes (p/n 11750). All one need do is unbolt the old pieces and bolt-in the new.
On a '68-'82, the nuts holding the power booster to the firewall are accessible from under the instrument panel. The upper left hand nut is a bitch to even see, much less get to. We wrapped two layers of duct tape around a 3/8s-drive u-joint for stiffness and "glued" the nut into a socket with RTV silicon. With that and an extension, he could easily get the nut on the end of the stud without having to actually reach the stud with his fingers.
If your power booster is a '65 or a '66, exchange units may be hard to find. Some large cities have rebuilders that may be able to recondition an existing unit. Also, Stainless Steel Brakes Corporation can rebuild your booster by mail. Lastly, if you're working on a non-power brake car, Vette Brakes & Products has a full line of rebuilt master cylinders going back to 1960.
Many people never give a second thought to brake fluid. Well, my good friends, with brakes a high profile service issue on the '65-'82 cars; we need to learn more about the stuff that looks like cream soda, feels warm on your hands and makes your Corvette stop.
Conventional brake fluid is related to antifreeze in that it is polyalkylene-glycol-ether based. Chemists say it's "hygroscopic" which means it also shares antifreeze's ability to absorb water. Where glycol brake fluid and antifreeze differ is that even a small percentage of water reduces brake fluid's boiling point. When the fluid boils, vapor bubbles form and act just like air in the system. Pedal feel degrades as does braking performance. Since the brake system's product is heat; it makes sense that you want as little reduction in boiling point as possible.
Brake fluid sold for road use in this country meets standards set by the U.S. Department of Transportation and the Society of Automotive Engineers (SAE). The specifications are categorized by minimum dry and wet boiling points:
A general rule is that, in a typical brake system, brake fluid will absorb about 2% water over a period of 12 months. Vette requested information on brake fluids from SAE. Included was a water content vs. boiling point graph for SAE's "RM-66 Compatibility Test Brake Fluid" which is an average DOT3 fluid mixed from three factory fill fluids and one aftermarket fluid. Two percent water lowers the boiling point of RM-66 to approximately 335Â°F and 3.7% (the SAE test standard for wet boiling point) drops it to 285Â°F.
In normal driving situations, such as downhill, freeway off-ramps or mountain roads, can heat brake fluid to 275-350Â°F at the hottest parts of your Corvette's brake system. If your old brake fluid has 2% or more water, localized boiling could occur.
Our advice is to change the brake fluid every 18 months regardless of mileage and do it annually if the car "lives" in a moist environment. As for what brake fluid to use; we have heard from several sources, including a couple at GM, that one of the best DOT3 fluids, reportedly because of low hygroscopicity, is sold by Ford (p/n C6AZ-19542-AA).
If you are seriously concerned about brake fluid contamination, there is a hand-held electronic device available that determines the amount of corrosive material in brake fluid. The battery-operated, Mac Tools Brake Fluid Tester (p/n BFT900) leaves no question as to contamination. It works on a comparative basis ie: you first "zero" the unit on a known good sample then test the fluid in your master cylinder. We use the BFT900 to periodically test brake fluid of magazine project cars along with that of personal vehicles owned by members of Vette's staff.
Bleeding the '65-'82 brakes can be a problem because of the system's large volume and its ability to trap air. Since you have to remove the rear wheels to get at the rear bleeder fittings; make sure the back of the car is jacked-up about a foot while you bleed. This eliminates air "domes" in the master cylinder and in the rear brake calipers.
There are four bleeding methods: gravity, foot, vacuum and pressure. Of those four, we don't recommend vacuum bleeding because we believe it can suck air past the primary piston seals. Obviously, if that happens you have a self-defeating process.
Gravity bleeding is simple and effective, but time consuming. One by one, open the bleed fittings for about 15-20 minutes and let them bleed through a hose into a bottle. Though you may spend an hour or so watching brake fluid drip, this method works well if you are alone. While you wait, read Vette Magazine, balance your checkbook, do your wash, or clean your cat box.
Foot bleeding is the most common technique. It takes two people: one pumping the brakes and the other bleeding. Space precludes basic instructionsâ€“see Service Manuals for that. We do have a few advanced tips: first, the "pumper" must carefully stroke the brake pedal. Rapid smashing the pedal to the floor is not acceptable. The pumper should take three, full strokes at moderate speed, with moderate pressure and pausing slightly at the end of each upstroke. At the end of third downstroke; maintain moderate pressure on the pedal and say "Holding." Once the three-stroke-and-hold cycle is complete; the pumper should wait about 5 seconds before beginning again.
At Vette, we prefer using a pressure bleeder. It is less time consuming and can be done by one person, so there is less chance for error. Mac Tools has an excellent unit (p/n BBT2) that is great for enthusiast use. It holds a gallon of brake fluid, has a built-in pressure gauge to prevent excessive bleeding pressure, a hand pump which eliminates the need for a compressor, and it works with any of Mac Tools' many master cylinder adapters (p/n BBT342G, for C3 dual cylinders; p/n BBT345G, C2 single cylinders). We have used the Mac unit for quite some time and it makes periodic brake bleeding a quick and efficient process.
After bleeding with any type of pressure unit, make sure the master brake cylinder is not overfull. You want a 1/8-3/16-inch air gap above the brake fluid. Lastly, regardless of bleeding method, while the bleed fitting is open, a few taps on the caliper housing with a plastic hammer will help dislodge air bubbles.
We stated at the beginning of this story that the '65-'82 disc brake system is temperamental. The ideas we've discussed here should help you find your way around most of the quirks.
see your local dealer
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