brake issues-long

[nut4crew]

Ok, here goes...
I neglected my '76 for a couple of weeks in my yard-I had planned to wash it and never got around to it. Anyway, when I finally started it to put it back on the driveway, the brake pedal slowly went to the floor, hit bottom, and the brake light came on. However, at the "normal" stopping point, the brakes would stop the car, even though I could continue all the way to the floor. I checked the level in the master cylinder, and it was low for the front. I added more fluid, n/h.
So I put it back on the driveway, and come search the forums. I go back and re-pump the brakes to look for noticeable leaks-none. I re-check the level in the master, same.
Then I realize something funky has been going on when I turn the car off. My left headlight (I didn't check the right, the hood was up) would go up, then come back down. So I put my ear to large item behind the master cylinder, and think I hear a leak. I'm assuming that's my power brake booster. Anyway, I turn the car on and off three more times, and hear a distinct leak, and the headlight thing, each time. I put my hand to the crease of the booster, feel air, and hear a decrease in air.
So the whole point of this is, what do I do? Should I start with the basic inspection of lines, bleeding the system, etc? Or do I go right to the booster, which is my inclination. I'm not seeing much information/troubleshooting in my Chilton's for the booster that matches my symptoms.
Any suggestions appreciated.

[SIXFOOTER]

OK, you have a couple of things going on: Check out the check valve on the vacuume line that feeds the headlight system, its probably bad, thats why the headlight acts up when you turn it off. Next is the Master Cylindar. It is probably leaking. Onlt real way to know is to bleed the brakes, do you know how to do that?
Next is the brake booster, it should hold vacuume when you shut the motor off and should not leak when it is running. Tackle it one item at a time.

[6880 Mike]

Brake boosters seldom fail. Look at other brake system issues before swapping out a booster. Time to bleed the brakes and rebuild/replace calipers. How old is the master cylinder? Might be time for a replacement. You obviously have a leak in the system regardless of whether the evidence has puddled up on the driveway.

[minifridge1138]

If you hear/feel air coming out from around the booster, then I would start by replacing the vacuum lines. It is much more likely that an old rubber hose has split than the booster has failed.

The fact that your headlights do funny things tells you that there is DEFINITELY something going on with your vacuum. Start there.

I don't know how a bad brake caliber would cause air to leak from around the master cylinder.

[JohnZ]

If the reservoir is full, there are no leaks, and the system has a decent pedal when you first step on it, then goes to the floor, the master cylinder is bypassing internally (failed seals) and needs to be replaced or rebuilt. The pedal going to the floor has nothing to do with the booster. Once you have brakes again and the car is safe to drive, then go after your vacuum leak(s).