Poor brake pedal on '80 Fairmont

[zeffer]

Ok here we go...
I bought this car about a month ago and it never had a very good pedal feel. While driving home the other day the pedal went to the floor while coming to a stop, thought master cylinder gave up. Bought a new master, bench bled it and installed it, cant get much pressure from any bleeders. Exchanged master for another thinking the first was faulty, still no pressure. Removed proportioning valve and took it apart and cleaned all the gunk out of it thinking that was the problem. Nope, still no pedal.
Here is what I've done so far:
->Replaced all flex hoses
->Blew air through all steel lines(no blockages)
->Compared new masters to original and all dimensions are the same
->Cracked lines at master while my son held pedal and doesn't seem to be pressure there either

When driving the car before, the pedal assist felt fine, just not a firm pedal. Something happened to just make the pedal go to the floor. Everything logically tells me it's a hydraulic problem, but could something have let go inside the booster possibly not letting the pushrod travel far enough into master cylinder?
Help guys(girls)!!

[gr79]

If there are no leaks;
Linings are good;
Parts are good;
There is air in the system.

[zeffer]

I have bled and bled the **** out of these brakes... there isn't a lot of pressure when cracking any bleeders, fluid does come out but not forcefully. Normally when you crack a bleeder on a caliper, fluid will shoot a couple feet at least, in my experience anyway. I have bled at least a liter(quart) of fluid through this system, still not getting a good pedal. I have the proportioning valve off again and I'm going to temporarily tee the lines together front and rear to eliminate it from the equation...
I appreciate any and all replies/feedback....never had an issue like this in any car I've ever done brake work on!
Jaymie

[Patrick Olsen]

Maybe unbolt the master from the booster, cycle the pedal, and see what the plunger on the booster is actually doing? (I don't know if it's "bad" to cycle the booster without a MC bolted to it, so if it causes the plunger to shoot out and kill the neighbor's cat, I apologize. )

You could also put some resistance on the plunger to see if you can get a feel for how much force the booster is generating - maybe (as you suggested) something failed in the booster and the thing just isn't generating as much assist as it should?

[zeffer]

I did have my son push pedal with master removed and everything seems ok, pulled pushrod out and can't really see anything abnormal inside. The only odd thing was when I was driving home that day...one stop light I have a pedal and next light no pedal. Something just gave up...everything leads me to believe that it was the master, but obviously not. I will be bypassing the proportioning valve on my next days off to see if that is an issue...

Thanks everyone for the views/replies to my post....any help is greatly appreciated!
Jaymie

[ukravit]

There is another way to check for air. Have someone pump up the brakes, hold the pedal , and you slowly crack the line that screws into the master cylinder.
Either fluid will seep out or it will push the air out. Bench bleeding is really hit and miss. Done this many times to get rid of spongy feel.

[zeffer]

Well I found my problem....
I was getting ready to put a match to the car and burn it to the ground (kidding) and I noticed brake fluid leaking from the right rear floor area near the torque box. I removed the lower back seat and found the line rusted through!!!!!! The underlay had soaked it up for who knows how long and it finally drained out the body plug. I repaired the line, bled my brakes and holy **** I have a brake pedal!!
I will never forget this....

[sleeper]

I was just about to suggest looking there. Min had no rust but you can see where it has rubbed over the years