RBeach

I also posted this is the GMC Jimmy forum, but figured I would also give this a shot here:

Brake pedal started slowly sinking to floor. I decided it was probably time to replace master cylinder. Bench bled new master cylinder (rebuilt Cardone) with bench bleeding kit twice (Raybestos one where the plasting tubings go back into the resevoir). I installed new master cylinder along with the SpeediBleed brake bleeder screws and a new check valve for the booster. Bled all brakes starting from RR to LR to RF to LF. Pumped pedal about 10 times per wheel and made sure I was seeing the new clean fluid coming out with no air bubbles. I also kept a regular check on the fluid level to make sure it didn't get too low.

But I am still getting the soft sinking pedal. If I pump my brake several times, it gets stiff, but then slowly starts sinking to the floor.

When vehicle is off, and I pump brake a couple of times, it gets hard then very, very, very slowly sinks about 1/3 of the way with steady firm pressure.

This is driving me insane. I am no master mechanic by any means, but out of the many of brake jobs I have done over the years, this is the first that I have had a problem with. Then again, they were older cars without ABS brakes.

I took the Jimmy to the dealership last week to have the 4WD vaccuum harness and rear seatbelts replaced, because there was a recall on them. While I was there I asked them if they could pressure bleed the brakes and use the GM scan tool to bleed the ABS unit for possible air trapped in the internal valves of that. Believe it or not, but the dealrship said they did not have the scan tool to do it. Hard to believe a dealership that doesn't have the tools to work on their own vehicles.

I know this iis a long post, but someone PLEASE HELP ME. My wife is ready to strangle me for driving her car all the time.

Rusty

ricksza

If you have already replaced and bled the master cylinder, and there are no other leaks from the wheel cylinders or lines, then the only other place would be the Brake Pressure Monulator Valve.

I have contacted GM directly and they have said that it can't happen, but I have had two cases where I found internal leaks. Sorry, no easy way to check other than replace the Brake Pressure Monulator Valve. After the valve is replaced and bled, the pedal will still feel soft. To release the air trapped in the valve, find some gravel and activate to ABS a few times. The brake pedal will return to normal.

RBeach

OK. Here is where I stand:

Figured I would take master cylinder back where I bought it and swapped out for another (just in case I got a bad one). Bench bled the bad boy unitl it was rock hard. I installed it on the vehicle, then bled all four wheels (RR, LR, RF, LF) and still no luck. I figured I wouls test the brake booster. I cut the enigne off and pumped the brake several times til it got rock hard and it did not sink at all. While holding pressure on the brake pedal, I cranked the engine, and the pedal sunk down to the floor. Several people has said brake booster could be bad. Correct me if I am wrong, but would'nt a bad brake booster cause a hard pedal? I am thinking possibly that the ABS unit has air in it. I checked all lines, fittings, wheel cylinders and calipers for leaks, and found nothing. I was told that if it was the ABS unit, I need to take it to the dealership to have it bled with their scan tool.

Any suggestions?

ricksza, Where is the Modulator Valve? Is it the valve located right below mster cylinder?

RBeach

OK. Here is where I stand:

Figured I would take master cylinder back where I bought it and swapped out for another (just in case I got a bad one). Bench bled the bad boy unitl it was rock hard. I installed it on the vehicle, then bled all four wheels (RR, LR, RF, LF) and still no luck. I figured I wouls test the brake booster. I cut the enigne off and pumped the brake several times til it got rock hard and it did not sink at all. While holding pressure on the brake pedal, I cranked the engine, and the pedal sunk down to the floor. Several people has said brake booster could be bad. Correct me if I am wrong, but would'nt a bad brake booster cause a hard pedal? I am thinking possibly that the ABS unit has air in it. I checked all lines, fittings, wheel cylinders and calipers for leaks, and found nothing. I was told that if it was the ABS unit, I need to take it to the dealership to have it bled with their scan tool.

Any suggestions?

ricksza, Where is the Modulator Valve? Is it the valve located right below master cylinder?

RBeach

Well the saga continues:

I replaced master cylinder (bench bled) and brake booster (along with new check valve) with new units. I then bled the brakes in correct order (RR, LR, RF, LF). I had a good solid pedal for a couple of days. Seemed a little stiff for my taste, but I could deal with it. Now it seems to have good solid pedal to stop at low speeds, then slowly sinks to floor at light. If I am going around 45 and need to slow to make turn, it seems to take more pressure with less stopping power to do this.

I had a friend who is supposed to know about brakes come by and take a look. He jumped the motor on the ABS unit to make it constantly run, and then bled the brakes. Wierd as it sounds, it still didn't sorrect the problem.

Any suggestions as to why the pedal is still sinking?

Rusty

ricksza

The pedal is sinking because there is a leak either external or internal. If you're losing fluid, double check all your lines, wheel cylinders and calipers. If they're okay, it must be either the master cylinder or ABS Brake Pressure Modulator Valve. Just because the master cylinder is new doesn't mean that it can't be bad.

RBeach

I have a stupid question:

Where is the ABS Brake Pressure Modulator Valve located? Are you referencing the combination valve under the master cylinder?

Also, I tried 3 separate master cylinders, 2 which were bench bled by a shop down the road. I am not losing fluid at all, which seems to be the strangest thing. The only thing I could think of was that the ABS hydraulic unit was going bad. I heard the Kelsey-Hayes sytems weren't the most reliable, but others have told me they like the system.

Another strange question: Do you think a caliper that is frozen/hung up could cause the same problem? I haven't pulled them from the rotors to check yet, but I have gotten under the vehicle, and noticed the previous onwer had the front brakes hoses replaced recently.

I am almost at the point of driving this thing off a cliff.

Rusty

RBeach

By the way, I should have given information about vehicle if I haven't done so already.

1991 GMC S15 Jimmy 4WD

ricksza

After three master cylinders, I think you can safely rule it out.

The ABS Brake Pressure Modulator Valve is the large (expensive) ABS unit on the left front fender well. It has two inlet lines and four outlet lines. What happens sometimes is the fluid leaks past the valves internally. This way the pedal will sink with no fluid loss. They are a pain to bleed.

A stuck caliper will not cause a sinking pedal unless it's leaking.

awc109

ricksza - Have you gotten anywhere with your brake problem? Sound similar to mine but I haven't gone the route of the booster yet. Trying to not go through the process of elimination by replacing parts.