Brake Problem?

[cshupe]

I have a 1997 Yukon and my brake pedal is very firm when the car is off and when the car is started the pedal slowly sinks to the floor. I have replaced the master cylinder (twice one reman and one new) the front calipers and the rear wheel cylinder bearings. I have bled the system relentlessly and as I said the pedal is solid as it should be but as soon as the car turns on it goes to mush. My brakes work fine but we have a 95 Suburban and I have never had this problem. Any suggestions?
Thanks,
Chris

 

[SunlitComet]

if you are saying the pedal sinks to the floor on it's own then that would be your vacuum brake booster behind the master cylinder. Something has gone wrong with the control mechanism inside.

 

[cshupe]

Brake problem

The pedal only sinks if I am applying pressure to it. If I am pressing on the pedal when it is firm and continue that pressure while starting the car it will sink slowly. I was under the impression that if the brake booster was bad it would actually be harder to press the pedal?

 

[SunlitComet]

Well it will sink down some if pressing it while you start it. That the boosting happening. The bad booster as you described can also happen. I was thinking the piston that moves inside when pressed might be stuck in a depressed state. If you have blead out the master cylinders before install(which of course the problem was there before you replaced them), calipers and wheel cylinders the only things that could be left are weak brake hose, incorrectly adjusted drum brakes, defective brake linings or air in the ABS unit.