Hard brake not stopping at low speeds?

Tiny
GUNNERGARIDEL
  • MEMBER
  • 2002 GMC SIERRA
  • 6.0L
  • V8
  • 2WD
  • AUTOMATIC
  • 346,000 MILES
A few weeks ago, I turned left out of a parking lot and did not exceed 10 MPH before I had to stop for a light. When I applied the brake, the pedal became very hard, and I felt some pulsating in the pedal, and I almost didn't stop in time.

I had wheel speed sensor and ABS module codes. I replaced both, but the problem still persisted when I start driving at a low speed and have to stop before I get up to road speeds.

Once I'm up to normal driving speeds, I don't have a problem stopping.

I had the truck 'scoped' at a shop. They told me there were no codes, but the reading showed my speed always stopped at 3 MPH and was never recorded as reaching 0 MPH indicating a full stop.

At first, I thought this was due to my oversized tires which caused my speedometer to read slower than my actual. I had a shop put on a set of tires 3 sizes lower than my actual tires, but the problem persisted, so I returned to the larger tires.

One day last week, it happened again. I was so close to the car in front I was scared and really pushed down on the pedal trying to stop the truck. When I did, I felt a release of pressure, and the brakes worked, and I stopped!

So now, I have two theories about this:

1. It's a hydraulic issue related to my Booster or

2. If the sensor believes I'm always at 3mph, then when I start a '0' and then have to stop before I make driving speed, the ABS won't work to stop my brakes. Or both theories are wrong, and it's something else. I'm open to any and all suggestions. Help!
Friday, April 18th, 2025 AT 5:00 PM

18 Replies

Tiny
CANNON1349
  • MECHANIC
  • 862 POSTS
Hello, it does sound like a booster issue. However, if the ABS sensors were replaced, I would be interested in what you see when you look into the hole in the knuckle. Many times, I've seen the wheel bearings fail without noise or play and leak grease into the spindle, causing an erratic signal and affecting stopping power. Below I will attach the instructions for removing the brake booster and add some pictures. Please let us know how it turns out.

Removal Procedure
1. Apply the park brake and block the wheels.
2. Remove the brake master cylinder.
3. Remove the vacuum hose from the brake booster check valve.
4. Remove the pushrod retaining clip (4) from the brake pedal stud (1).
5. Remove the stop lamp switch (2) and the pushrod (3) from the brake pedal stud (1).
6. Remove the 4 vacuum booster mounting nuts.
7. Remove the vacuum booster.
8. Remove the gasket from the booster and discard.

Installation Procedure
1. Install the gasket to the booster.
2. Install the vacuum booster.
3. Install the 4 vacuum booster mounting nuts.
Tighten the nuts to 36 Nm (27 ft. Lbs.).
4. Install the pushrod (3) and the stop lamp switch (2) to the brake pedal stud (1).
5. Install the pushrod retaining clip (4) to the brake pedal stud (1).
6. Install the vacuum hose to the brake booster check valve.
7. Install the master cylinder.
8. Release the park brake and unblock the wheels.

William
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Saturday, April 19th, 2025 AT 6:43 PM
Tiny
GUNNERGARIDEL
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  • 32 POSTS
William: Thank you very much, and I will keep you posted. Today, I was telling a friend about the truck, and he made two suggestions:
1. He opined the problem could be a pitted rotor because he had a similar problem; and
2. I should bleed my abs brake system or brakes.
Do either of these make sense to you, based on what I described?

By the way, I've been paying very close attention to this, and I've noted as my speed approaches -0-, on a gentle stop, it happens if I press too hard on the brake pedal. Any thoughts?

Dudley Garidel
Baton Rouge, LA
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Sunday, April 20th, 2025 AT 12:15 PM
Tiny
CANNON1349
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Hello, I would be interested to know what the ABS codes are in the memory. What you describe reminds me of the issue I mentioned about the wheel bearings. If at any point the speed sensor picks up an erratic signal, the ABS module will think you are slipping and will engage, causing a symptom that you are describing. I would definitely take a look inside the knuckle / hub and see if there is anything causing a signal interference before going down further avenues. Usually, an ABS system bleed will have to be done with a scan tool with that particular capability.

Please let us know,
William
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Sunday, April 20th, 2025 AT 8:00 PM
Tiny
GUNNERGARIDEL
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  • 32 POSTS
Sir, I'm glad you wrote, because I was going to write today, to give you an update. Previously, I replaced the ABS module and wheel speed sensors. As of two hours ago, I replaced the hydro boost. I noticed a slight improvement; however, it's still doing what it was doing previously.
I tried to replace the ABS hydraulic pump, but it was defective [bought from recycle business]. The AES certified mechanic was unable to bleed the system.
At this point, I believe I should replace the wheel bearings, unless you have any other ideas.
I cannot tell you how much I appreciate your help!

Dudley Garidel
Baton Rouge, Louisiana
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Monday, May 12th, 2025 AT 4:30 PM
Tiny
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By the way, other than this, my truck is running like new :)
Dudley Garidel
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Monday, May 12th, 2025 AT 4:31 PM
Tiny
CANNON1349
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Hello, thanks for the update. Before you remove the wheel bearings you should look inside where the ABS sensor rides, and if it is full of grease this could be your answer. I truly hope the wheel bearings fix your issue. Please let us know how it goes!

William
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Monday, May 12th, 2025 AT 6:33 PM
Tiny
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Sir,

If you are correct about the excess grease, where the ABS sensor rides, being the issue, what would I need to do to correct it?

Dudley Garidel
Baton Rouge, LA
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Saturday, May 17th, 2025 AT 3:23 PM
Tiny
CANNON1349
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If there is grease blocking the sensor signal it is because the wheel bearing assembly has failed and is leaking grease. This is especially prevalent in GM vehicles. Even if there is no wheel bearing noise or play, the bearing seal has failed causing grease to build up in the spindle. So the only fix would be to replace the wheel bearing assembly.

William
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Saturday, May 17th, 2025 AT 6:53 PM
Tiny
GUNNERGARIDEL
  • MEMBER
  • 32 POSTS
Sir,

This makes total sense to me. I will be looking into this and will contact you with the results.

Dudley Garidel
Baton Rouge, LA
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Sunday, May 18th, 2025 AT 11:55 AM
Tiny
CANNON1349
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Please let me know how it turns out. Nevertheless I am here to help when you need it.

William
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Sunday, May 18th, 2025 AT 12:59 PM
Tiny
GUNNERGARIDEL
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Sir:

It's been over a month, so I thought I should bring you up to speed about my situation. To say things have been maddening is an understatement. As of today, the following repairs have been made since my last letter to you:

1. Changed wheel speed sensors from Dorman to AC Delco. When I made the change, I used my Dremel to grind down the hub a bit to ensure the surface was smooth as new.
2. Installed new speed sensor.
3. Installed a brand-new ABS control module as of two days ago.
4. I believe I mentioned [but I'll repeat] I replaced the hydro boost about 3 months back.

By the way, all the brake pads are new, installed about 5-6 months ago.

The good news is the truck stops, which is important! The bad news is the issue with the ABS pulsating when slowing down still exists. It happens at the exact moment the truck speed slows to 10 MPH when braking, every time, unless I'm feathering the pedal.

At this point, the only thing I can think of doing is replace both front wheel hub assembles, unless you can suggest something else? If you can think of anything else, I'm all ears and eyes! ;)

Dudley Garidel
Baton Rouge, LA
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Monday, June 30th, 2025 AT 6:27 AM
Tiny
CANNON1349
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Hi,

When you replaced the sensors did you check inside the hole for excess grease? This will cause a breakup in signal. This is a very common failure with GM products. I would replace the bearings and make sure everything is dry in the knuckle and drive it.

William
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Monday, June 30th, 2025 AT 8:47 AM
Tiny
GUNNERGARIDEL
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Sir - thank you for that information. I will definitely look at this.

I did, however, obtain some information this morning which I thought you should know. I have an acquaintance who has a shop and he told me to drop by one day and let him run a scan on the abs system. I had some time this morning and stopped by. He ran a scan on the ABS and printed the report for me. What I saw stunned me.

According to the report, all THREE wheel sensors show my truck was moving at 3MPH, even though my truck was sitting in Park and the engine was NOT RUNNING!

Obviously, something's wrong, but what? Any ideas or is this related to the bearing & grease issue?

Dudley Garidel
Baton Rouge, LA
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Tuesday, July 1st, 2025 AT 8:35 AM
Tiny
CANNON1349
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Hello,

This can be considered electrical noise from the voltage fed from the ABS module. The real test would be to see how the sensors react to spinning the wheel and slowing down, you might be able see the erratic signal. These sensors have a magnet that pick up the teeth on the tone ring of the wheel bearing, and so this is how it knows how fast you're going. When something obstructs that, the ABS module will perceive the break in signal as the vehicle slipping (like on ice or hydroplaning) and will activate the ABS. I would still be interested to see how these bearings look, and, more times than not, you purchase the bearing with a sensor already calibrated to it.

William
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Tuesday, July 1st, 2025 AT 7:54 PM
Tiny
GUNNERGARIDEL
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Sir:

I agree with you. I believe the bearings are the problem. That will be next. Thanks for all your help in this.

Dudley Garidel
Baton Rouge, LA
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Wednesday, July 2nd, 2025 AT 5:43 AM
Tiny
CANNON1349
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No problem, I'm eager to hear what happens next. Thanks for the update!

William
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Wednesday, July 2nd, 2025 AT 7:29 PM
Tiny
GUNNERGARIDEL
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  • 32 POSTS
Sir:

I'm pleased as punch to report the issue is finally resolved. I bit the bullet and replaced the two front wheel hub assemblies and rotors! Apparently the problem was the hub assemblies were just worn down since I have 346,700+/- on them. Considering they held up for 23+ years, I really can't complain.

The repair shop owner did tell me the bearings were pretty much shot to pieces, so no surprise there. They did a thorough scan on my truck brake system when they finished and discovered the ABS module was showing I had four different tire sizes in the module system! This means the last shop who worked on my brakes did not really do the ABS scan they claimed to have done. Oh, well. :(

I can't thank you enough for all your advice and responses! You helped me learn a lot just by causing me to 'look'! : )

I have a NEW truck again, even if all the original parts have been replaced, and I only had to wait 23+ years! ; )

Dudley Garidel
Baton Rouge, Louisiana
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Friday, July 11th, 2025 AT 5:32 PM
Tiny
CANNON1349
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Hello,

It is great to hear from you about this success. Indeed, the wheel bearings tend to be the culprit on the GM products. Please feel free to use the website anytime, and tell a friend! We're here when you need us.

Thanks,
William
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Friday, July 11th, 2025 AT 6:42 PM

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