Trouble-Shooting Brakes on a Hummer

Repairing Brakes Article

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Problems:


Don't forget to adjust the parking brake cable tension. Too much tension will cause the rear brakes to drag.

Check for oil residue on or around the rotors that might be burning and causing a smell. This could be a bad seal on the output shaft of the differential that is leaking oil on to the rotors and causing the burning oil smell. You might find that there is oil on one side causing the other to work harder to stop the truck and overheat the rotor. Check the level in the brake fluid reservoir. You could have a leak in the hydraulic system. Brake fluid maybe leaking and burning on your rotor.

A proportioning valve balances the braking pressure between the front and the rear. A defective valve will cause problems with fronts or rears locking up. 1992, 1993 & 1994 all used proportioning valve p/n 12339353 Then in 1995 & up to ABS(1999) , proportioning of the valve was changed and p/n 6005728 was born. The original valve proportioned the brakes around 50/50. The newer part gives more bias to the front which is more desirable for a lightly loaded civilian truck.

While you're at it make sure that you are using the proper brake fluid. All trucks up to and including 1998 use DOT 5 Silicone fluid. All 1999 and newer ABS TT4 trucks use regular DOT 3 fluid. They do not mix and will cause problems if they are.

You also could have a frozen piston in the caliper which is overheating the rotors. This heats up the output shaft and can cause the seals to go bad from too much heat.

If you have a 1999 or newer truck it will have ABS. If the ABS system is defective it could cause one wheel to brake more then another.


Parking Brake Spring and Cable Clevis

Another common problem is the parking brake hanging up which is of course on the rear only. It could hang up at the caliper or in the cable. Make sure that the caliper and cable are free when the brake is released. Check to see if the return spring for the parking brake is present or corroded on the caliper. It is located on (around) the same shaft where the parking brake cable attaches. Its function is to tension the cable. If it has broken you might have some dragging. You may have to remove the caliper and disassemble and clean it up. I had to replace my parking brake cables. It is a major job.

 


Parking Brake Spring and Cable Clevis

Another common problem is the parking brake hanging up which is of course on the rear only. It could hang up at the caliper or in the cable. Make sure that the caliper and cable are free when the brake is released. Check to see if the return spring for the parking brake is present or corroded on the caliper. It is located on (around) the same shaft where the parking brake cable attaches. Its function is to tension the cable. If it has broken you might have some dragging. You may have to remove the caliper and disassemble and clean it up. I had to replace my parking brake cables. It is a major job.


Parking Brake Cable looking Back

If you do take the caliper apart check the piston and cylinder for corrosion. Replace any bad parts. If the cylinder is bad you will have to get a new caliper.

Decreased Brake Pedal Travel or Slow Return

  1. Check for a worn brake pedal return spring. Replace if worn.
  2. Check brake pedal bushings for signs of wear or binding. Replace if worn and lubricate as needed.
  3. Check for kinked or damaged brake lines which may restrict brake fluid. Replace any damaged lines.
  4. Check hydro-boost. Depress brake pedal several times, with engine off, to exhaust accumulator pressure. Depress brake pedal and start engine. Brake pedal should fall, then push back against operator's foot.
    1. Perform pressure test
    2. Replace hydro-boost if not operating properly.
  5. Check brake calipers for binding as a result of corrosion or dirt. Check brake rotors for free movement. If rotors do not move freely, remove calipers and clean caliper guide pins.
  6. Check parking brake cable (rear brakes only) for proper operation. Repair as required.

 

Brakes Lock up and Stick

My front brakes seemed to lock on and stick. Both rotors are very hot and the brakes seem to be dragging with a chattering sound

I had a similar problem - except it was primarily the rear brakes, although I found out later that the fronts were dragging as well just not as bad.

After several mis-diagnoses the problem ended up being the brake master cylinder sticking internally - keeping pressure on the brakes, even when the foot was off the brake. Once that was replaced no more problems. This problem is also caused by a failing hydroboost. I did have to replace the cooked rotors and pads at both ends though.

Brakes Lock up in the Front or the Rear

This could be due to a failing proportioner valve. This valve controls the amount of brake force that goes to the front and the rear. In the case of a 92 thru 94, you may want to try upgrading to the newer ('95 thru '98 10,300 lb.GVW) brake proportioning valve, AMG p/n 6005728.

I hear a 'slapping' sound from the brakes

Check the brake caliper guide pin rubber sleeves.  If they're worn, the whole caliper will slam up against the stop and make a rather loud clacking type sound when first applying brakes.

I hear a clicking sound.

The brake pads have a small amount of vertical movement the way they are mounted.  If your rotors are even slightly warped, they will lift the brake pads and then allow them to drop, causing a metallic click sound.

Diagnosis:
* The noise is coming from the front brakes (when driving forward).
* Slight pressure on the brakes will stop the noise.
* Driving in reverse will stop the noise (but may cause the same noise in  the rear).

It's normal.

I feel Pulsation in the Brake Pedal

Warped rotors will cause pulsations in the brake pedal during braking, and cause variations in braking effect at low brake pedal pressure If the rotors are warped more than a small amount, they should be replaced.

Over the course of the day my brakes fade and the pedal gets soft

Water sometimes manages to get in past the piston seals on a caliper. It will stay in the caliper (were it DOT-3, it would be absorbed, and end up all over the system) Your truck has DOT-5 fluid, which will not mix with water. what happens is that when water gets into the system, it ends up staying as a puddle in the fluid.

If the caliper gets hot enough to boil the water, you will see exactly what you have, hard brake effort, brake pedal goes down excessively, and poor braking performance.

Bleeding the brakes will not remove water in the system with dot 5. Because water is heaver than fluid it will sink to the bottom of the caliper while the bleed is at the top. If there is water in the calipers you should take them apart and clean them out.

ABS light Staying On

The ABS/ TT4 lights in my 2001 wagon stays on. I shorted pin 4 to 15 on the data link connector (DLC) to get it to read out the fault code. The DLC is the plug that you use to hook up an electronic scan tool. You can also plug a scan tool in to get the code. I got a 2 - 1 ECU internal failure; replace ECU.

I replaced the ECU and everything works fine now.

 

Pictures by Chuck Kopelson

Caliper Rebuild Manual Pages