Frustrating brake problem 91 MJ

[JeepZJ]

Here it goes:
Got the Jeep from my brother in law and he replaced the MC and rear wheel cylinders. Brakes felt bad like they needed to be bleed. Did all that and still had a soft pedal. Changed MC and eliminated the rear prop valve, bleed and pedal had good initial bite but pedal goes to floor with no additional stopping power. Put in a front prop valve from an XJ and swapped the MC again, bleed with a vaccuum bleeder and the pedal still feels like it has good initial bite and then fades. With the engine running, I push the pedal slow and it goes almost to the floor. If I stab it, it feels pretty good. If I pump the brakes, it feels better but I have bleed it all ways I know and no air is coming out. I swapped out the calipers with some off of my ZJ I had and no change. Yes the bleeders are up. It almost feels like when I first push the pedal, it needs to take up some slack in the system. The rear drums appear to be adjusted right and it does not change the pedal feel if I clamp off the rear hose or any of the front.
I am a pretty competant mechanic but this is stumping me. Any help is appreciated. Thanks!
Nathan

 

[montanaman]

To test the "slack-in-the-system" theory, you can pump the brake pedal with the hand brake fully engaged. That will take ALL slack out of the rear shoes. If the pedal firms up nicely with the hand brake engaged, that's a sign you need to adjust the rear shoes more tightly.

I adjust them with the wheel off. I usually over tighten them so the drum won't turn. Then I back them off until it just turns. Then I pump the brake pedal to help seat the shoes, then I can usually tighten the adjustment a little more.

Good luck.

 

[Jess]

did you bench bleed the mc?

 

[JeepZJ]

I have held down the e brake lever and it did not make any difference. It seems like it is the front calipers that are giving me problems.
I did bench bleed the mc. Even if there was a little air left in, wouldn't the vaccuum bleeder pull out the air?
Nathan

 

[Eagle]

It's most likely the proportioning valve that's creating the problem. Was the XJ unit you swapped in new or used? If used, did you open it up and clean it out thoroughly before installing it?

 

[JeepZJ]

yes it was used. i did clean it a little but did not open it up. what should i look for? thanks
nathan