This question involves my '91 S-series truck but I thought I'd check with the brake gurus here.

I have a pedal that goes soft. You stab the brakes, it stops, then the pedal sinks slowly to the bottom. I basically loose the rear brakes. Here is what I have done so far.

!. Replaced the master cylinder. Twice. Thought I may have gotten a bad one.

2. Replaced the vaccum booster.

3. Replaced rear wheel cylinders and the flex hose.

4. Bled, Bled, Bled some more. I can't see how there could be any more air in the system.

This weekend I am installing rebuilt calipers and new flex hoses. Tha way I will have replaced nearly EVERYTHING in the hydraulic system.

I have been told my ABS box/proportioning valve is bad, and that it's about a $300 item. I have looked at my fiance's Camaro and it appears to have the same crappy ABS system.

If this is so I am going to get a non-abs proportioning valve and switch it over. They never worked well anyway.

Any advice?

Eric

 

well My friend I fear you are on the right track.

 

Um, 91 camaros don't have ABS. I own one.

It sounds as though you have a LEAK in the system somewhere, and not necessarily a bad component. The pedal should NOT sink after initial application.

 

Your fiances camaro (if it is a 91) does not have ABS brakes. Third generation f bodies never came with antilock brakes.
I think you are talking about the proportioning valve that is just under the brake master cylinder. I don't think that will be your problem unless it it leaking. If your brake pedal goes to the floor like you say it does, you must have a leak in a brake line somewhere. When you leave your foot on the pedal, the reason it sinks is because you are reducing the pressure in the brake line as fluid leaks out. I would first look at all the lines and make sure the don't have any brake fluid on them, cause they shouldn't. Just getting another proportioning valve will not help because the problem will still be there, plus you have to re bleed the brakes.

 

THe problem is actually with my '91 S-10 which has antilock brakes, not that they worked well. There is NO external leakage anywhere.

Still I will replace the calipers and hoses this weekend in an attempt to cure this thing but I'm not holding my breath!

 

Been there. I never did find the problem on my 88IROC, but my plans were to replace the brake system with a Baer setup.

Before this I had rebuilt calipers and cylinders, replaced master, checked for leaks and bled, bled , bled. The symptom was the car would stop, but poorly (meaning my 87 Caprice stopped better), and if you put with pressure on the pedal it seemed like (given enough time) the pedal would go to, or nearly to the floor - a very uncomfortable situation.

My brother had an 89 Chevy truck (compact truck of some sort) - same problem. Replaced the master and some improvement but the same sort of "feel".

I wrote it up as an inadequate design - when I replaced the system on the IROC with Baer 13" track up front and 12" sport in the rear (lines, calipers, rotors) the hydraulic system is rock solid (and the car stops). I guess it could have been a line but a close inspection of the old lines didn't reveal anything.

I had previously looked at brakes as simple but this problem really stumped me.

 

Yeah, I never had problems with other vehicles either. I did dozens or brake jobs on other stuff and unless I used cheap parts never had a problem like this.

Eric