GMC ABS brake help request

[Briant]

I have a 1992 GMC Suburban 1500 4 wheel drive with 215,000 miles. Her name is Big Bertha. There is also a trailer brake device installed from the previous owner, which I have no need for and not sure if may be contributing to my problems below.

I have a few ABS troubleshooting questions.

1. When I first start the engine and begin to drive, I always hear a humm for about 3 seconds from under the hood where the ABS "control box" is located. After that it doesn't make the sound again until I turn off the motor and start it up and drive again. Is this an indication of a specific problem?

2. A few times I have been driving in the rain and when I approach a stop light, the ABS seems to kick in and I hear the humm again and the vehicle feels like it is employing the ABS to stop. This has happened even when I am not braking hard and ends up taking farther to stop than I would expect. Fortunately, I have not had a vehicle too close in front of me to cause a wreck, but it has been a bit shocking!

3. I replaced the front brake pads and rotors yesterday, thinking that the worn rotors may be causing false ABS readings. After completing the work and taking a test drive, I still hear the humm at 10 MPH for about three seconds. In addition, after completing this maintenance and bleeding the lines from all 4 tires, I still have a soft pedal and if I hold my foot on the pedal for some time, it drops toi about 1 inch from the floor board. The manual said to remove about 2/3 of the fluid from the master cylinder, which I did, to make room for the fluid returning from pressing the piston back into the caliper, but after doing this, no extra fluid was apparent in the reservoir. I bought this truck used and did notice that the fluid that I removed from the master cylender was grey to black in color. Do you have any ideas about why I am experiencing the soft pedal and why it would drop a little more after stopping if I keep my foot on it? Should I try flushing all the fluid through the bleeder valves with new fluid? Any diagnostic ideas would be greatly appreciated. I thought it could be the calipers, then found this forum and thought I would seek some outside advice first.

Thank you to anyone with some ideas!

[mudseeker]

1.
the noise you hear is perfectly normal. your ABS system is just performing a test of all the solenoids and pumps. it will do this test at every drive cycle, meaning every time you turn the key and reach a certain speed you will hear this noise.

2.
i would check your wheel cylinders in the rear for any leaks and also look at your axle seals. if they are leaking then they can contaminate your brake shoes and cause them to grab when force is applied. when the rear brakes grab, the computer will sense the difference in speed and activate the ABS system.

3.
a soft and dropping pedal would be a sign of a leak and/or air in the brake system. this is where the wheel cylinder comes into play again. the seals in the master cylinder could also be bad and allowing fluid to travel past them. master cylinder seals are harder to detect since the leak is internal and you will not notice a loss of fluid. i would start by replacing that old fluid first, it sounds like it is way past its usefulness. once you have the new fluid in the system, see how your brakes react and go from there.

[Case 7230]

I agree with everything MS says. The next time you are braking in the wet and hear the ABS come in press harder on the pedal. Also the next time you change the pads and are about to retract the caliper piston open the bleed screw and allow the piston to pust the fluid out into a container. Close the screw before you remove the clamp. This will prevent old heated fluid being pushed thru the ABS valves. The pedal sinking down with sustained pressure sounds to me like you have air in the system and you need to bleed it again. You can replace the old fluid with new at the same time.

[Briant]

Thank You both!

I was hoping that the buzz was just the ABS calibrating or self testing as you stated. This is good to know!

I will check the rear cylinders for leaks, flush all of the old fluid till I get nice clean fluid and remember the trick about using the bleeder valve, instead of the master cylinder, as directed by the book, to push out the old fluid in the future.

Thank you for your valuable responses! I will update this link with my results after performing your suggestions. By the way, this is my newer vehicle... my other is a 69 Camaro, which I understand, but haven't had much exposure to these ABS systems.

Thanks Again!

[Case 7230]

I love the `69 Camaros. Many years ago (more than I care to remember) I hunted down a low milage `69 Camaro 396SS in black. I left an offer on it and went back two days later to give him the asking price but he had sold it. I wish I had it now! Anyway good luck with both your cars.

[Faster4344]

Nice my old man has an all origional 70 nova 396/375ss!! But I do love those 1st gen camaros. hes go everything but a camaro

[Case 7230]

That is another car I wish I had owned. Outstanding.

[Briant]

Thanks again Case and Mudseeker. I did as you recommended and still had a pedal that would drop if pressure was continually applied. I bought a remanufactured master brake cylinder, replaced it, which was very easy and now I can stop Big Bertha. This forum is great!

[mudseeker]

glad to here you were able to solve your problem.

[CharlyG]

I love it when a plan comes together! H/T to the A Team!