Is there a comprehensive discussion on this somewhere? I've searched the threads and find basic generic brake diagnostic info, but it's not at all helpful to me. Saw the post this week on check valve, but I doubt that will solve sinking pedal part. My best guess is the RWAL valve, but I've not been able to find any posts where this solved the problem.

My discs, pads, drums, and shoes are in good shape. No external leaks at all. No fluid loss. Antilock does function OK. Braking power is OK, just a bit delayed due to pedal travel.

My symptoms:

1. Excessive travel. I've adjusted the rear multiple times and they are plenty tight. Applying parking brake does not change the amount of pedal travel.

2. Spongy feeling. Power bled system repeatedly (inc RWAL valve). Still spongy.

3. Slowly sinking brake pedal til AL and Brake warning lights come on. I've replaced the MC with a rebuilt unit which behaved exactly like the original. Then replaced it again with a brand new one. Each time carefully bench bleeding and then power bleeding the heck out of the system, including RWAL. As stated above, there are no external leaks and I don't loose fluid.

Any advice specific to the gen 1's would be appreciated.

Thanks.

 

I would throw a dump valve at it at this point. If the MC is new and nothing is leaking, the dump valve is the only thing I can think of.

 

Dumb valve or whatever. What is it, I have the same problem. Engine off have great brakes and yes it is new booster. ..

 

I ended up bypassing my dump valve, just cause im not a fan of the older abs units.
BUT, when i had your problem, I put the brake pedal to the floor overnight (or as long as you can) with my 1 1/2 wrench with the battery disconnected, and it was 80% better. After I drove it for a while i noticed my drums werent adjusting proper, adjusted them and got my other 15%. I think a power bleed would do the rest.

I know what your thinking "hows that gonna work?" I thought the same thing, but i figured it was worth a try, since the thing wasnt working anyways, the worst that could happen was blow up some lines (or find a leak), but it worked. Old mechanics trick from what I hear....

 

Just wondering has anyone solved spongy/sinking pedal by changing the RWAL dump valve?

 

It seems that, over time, the dump valve gets stuck in dump mode, cutting fluid travel and pressure to the rears, and making for worse brake performance. Maybe a flush every other year could help alleviate this?

 

It has been my experience that you cannot power bleed these brakes and get them right. Do it the old fashion way.