Spongy pedal.
Over the past year, I've replaced:
Master cylinder - fluid was black and it was leaking back into the brake booster.
Brake booster - master cylinder destroyed it.
Rubber hoses - installed new braided hoses.
Calipers - rears were frozen and only one front worked. Had collapsed hoses.
I have absolutely no air in the system. I've bench bled the master cylinder several times. I've bled the brakes God knows how many times. Today I checked again. I unhooked the master from the booster, tilted it forward till it was level and diddled the piston. No air bubbles came up through the holes in the bottom of the reservoir. Heard you could do that, so I tried it.
Last week I found out my cam is only pulling 12" of vacuum. Bought a reserve canister.
The brakes still go almost to the floor before they grab. Panic stops are pretty much out of the question. They do grab a lot better now since installing the new calipers and hoses last week, to the point where it doesn't take near as much effort to press the pedal down anough to hold the car. Before, I'd be pressing hard as I could and the car would still creak forward.
The brake booster hisses when I hit the pedal, has since I installed it. With the car off, I pumped all the vacuum out and held the pedal down, started the car, and the pedal sank to the floor.
The canister holds vacuum when I shut the car off. However, after sitting for about 15 minutes or so, it's about gone. Are boosters supposed to leak down like that?
I'm at a loss. The car's good enough to go drive around and I feel tons safer with the brakes in the condition they're in now. I just want to get this resolved.
The only thing I have NOT touched is the proportioning valve.
Can anyone help me?

 

the first thing i would do is seperate the master cylinder from the rest of the system. on the car may be difficult with braided hoses unless you can find caps for the steel lines leading to the hoses. a bad booster would give you a hard pedal as opposed to a mushy one. you may check your booster rod where it connects to the brake pedal. make sure the adjustment is not slightly engaged with the brake pedal in the relaxed position. assuming you have pressure to the rear wheels than i cannot see a prob with the proportioning valve. if you can seperate the master from the rest of the system, pump the brake pedal. if it gets rock hard the release one wheel at a time and repeat. good luck.