brakes sink

[profquad]

I've been working on this a while and am out of ideas..
The pedal goes to the floor but can be pumped up. The problem resets upon acceleration.
Replaced the master cylinder, no change. Bled the brake lines many many times. Not seeing any drop in the resevoir over time so I think theres no leaks.
Replaced both wheel cylinders and cleaned off the flywheels. But that's where the problem really is i think.
I've noticed the flywheels reset over time. Not sure why. There is some uneven wear on one of the shoes, so I thought wheel bearings? But when I brought it to a shop for new bearings they said the were fine and just adjusted the flywheels, which I told them I had been doing but whatever. A week goes by and the brakes sink again.
Just to be clear, adjusting the shoes via the flywheel works, but only temporarily.
And just today, to top it off, a loud metal grating sound started in the rear passenger side while driving, not while braking. Theres a good deal of shoe powder in that drum, and a rough texture in one section.. this is new.
What could be causing this deeply troubling behavior?

[Dr_Snooz]

What you describe is pretty typical for master cylinder failure. It's not unheard of to get a bad one from the parts store either. If your problem was EXACTLY the same after putting the new one in, however, that would suggest that the problem is elsewhere. It's possible your power booster is causing the problem, but that usually causes a rock hard pedal feel, hissing inside the dash, or something else. It's important to bleed the brakes in the sequence described in the manual or you could still have air in the system.

If your rears are not engaging correctly, that can cause real squirrely pedal feel. The rear drum brakes are marvelously complicated contraptions. There are loads of springs, clips, star adjusters (not flywheels) and whatnot that must be cleaned, lubed and replaced at regular intervals. It's time to dig into your rears and see what's up. Pull the whole mess apart, scrub it all clean, carefully inspect each piece, lube it as shown in the manual, have the drums turned and put it back together properly. Replace any bad springs, clips or other hardware. Better yet, replace it all just to be safe. Then adjust the brakes as described in the manual and you should be back on track.

Let us know how it goes.

[InAccordance]

If the brakes can be pumped up, I would think that's a bad booster.
Could be that check valve in the hose running from the intake as well.