91 Jimmy Brake HELP PLEASE!

I know this has been discussed before, but no two problems ever seem to be the same for everyone. Here we go:

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

More than likely the booster is shot..

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?

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?

I am posting also in Blazer forum in case anyone who owns/works on Blazers have ever run across same problem.

Rusty

Yes, a bad booster will cause a hard pedal, as will a vacuum leak in, OR TO, a good booster.
If it's an ABS problem, I'll be surprised, unless you've got different sized tires, even such as one 245 with 3 235's.