ABS is coming on? - Brake pedal judder?

First lets start with the basics. Be sure the rear shoes toward the front have the shorter linings and the rear shoes toward the rear have the longer linings. Next, if there was any grease or other contaminants on the linings or drums, that has to be washed off with brake parts cleaner before they go through the first heat cycle. Once the drums and linings get hot from normal braking, those contaminants seep into the linings and the drums or rotors, which are porous, and will never come out. That will cause chattering, and the only way to solve it at that point is to replace the linings and drums.

Check out this video to help get the codes

https://youtu.be/rTtAnsOlZU4

and

https://youtu.be/Pb6nXV-iamI

It sounds like your parking brake system is working okay, but nevertheless, look at the tops of all four shoe frames and be sure they're hitting the large anchor pins at the top of the backing plates. If they aren't touching that pin, the parking brake is partially applied due to a sticking cable. Next, use your thumb to push the parking brake strut bar forward against the pressure of the anti-rattle spring. That bar should have a good 1/8" of free play. If it is tight, the parking brake is stuck partially-applied. That will cause that brake to grab and possibly lock up under light brake pedal pressure.

New linings can be bonded slightly off-center relative to the old ones. That will cause them to ride on the ridge of rust near the outer edge of the drums or on the rust on the inner edge. We cut that rust off when we machine drums for reuse. You would assume that isn't a problem when you install new drums, but it's not uncommon for those to be warped a little. A light machining will take care of that. If they're egg-shaped, you'll get a pulsing brake pedal and a sensation of stopping, then not stopping, twice per wheel revolution. You won't feel that in the steering wheel.

You aren't going to bend a rear axle, but it's easy to get the same feeling if there's debris stuck between the flange and drum. This is common on front rotors that slide onto the bearing assembly. There are access holes in those flanges, and water sprays through them and forms rust spots on the backside of the rotors and drums. Those rust spots have to be scraped off before the part is mounted on the brake lathe, otherwise a warp will be machined into it. If the part isn't machined, it can be installed in a different orientation than it was before, and that puts that rust spot in between the flange and drum. You'll see that by watching the wheel as it rotates while you run the vehicle in gear and raised off the ground. We use a dial indicator to measure "lateral run out" at the wheel's lip where the weights go. .045" run out is excessive and is enough to cause a noticeable vibration, but if there's rust or other debris stuck on the mounting flange, you'll find a lot more run out than that.

Another common cause of warped parts is failure to use a click-type torque wrench when tightening the lug nuts. Guessing leaves the nuts tightened to different amounts with uneven clamping forces. One heat cycle or one splash of rain water on a hot rotor or drum will cause warping. This was never a problem up to the '70s, but it's real important on light front-wheel-drive cars and any other vehicles that have slide-on rotors.

I like to put new drums and rotors on the brake lathe and give them a light cut. Every once in a while we run into a part that was handled roughly in shipping, but another issue to be aware of is with Chinese parts. There is nothing wrong with their quality, but when we make parts out of cast iron, we set them aside to age for 90 days before they get their final machining. The Chinese cast 'em, pack 'em, and ship 'em, then they age on your vehicle. It's common for them to warp a little after a few months. One light machining will take care of that when it occurs.

It's important to use high-temperature brake grease on the six "lands" on each backing plate that the shoes ride on. This is especially important on GM vehicles because they are one of the few that don't have little tabs bent over to form a sliding surface. They just let the edge of the shoe frames grind on those lands, and before long grooves form in them. Those grooves can cause a shoe to catch and not apply under light pedal pressure, and in some cases, not release immediately. That usually shows up when new shoes are installed since the linings are thicker than on those that were removed, so the shoes are readjusted to accommodate that. That sets the shoe frames in a different location relative to those grooves.

Dandy. Happy to hear it's solved. Remember that if a pulsation comes back in the next few months, don't replace the drums or rotors. Just have a light cut taken on them. You should only have to do that once.

You have an ABS speed sensor that is not reading accurately at low speeds. It's a common problem with those truck. It is incorporated into the hub bearing assembly and the whole unit has to be replaced. The bearing is often the cause anyway.

That is the ABS activating because one of the wheel speed sensors is not reading accurately at low speeds and the computer reads that as a brake locking up. This is a pretty common problem on these trucks and there are a number of TSBs on the subject noting various causes like bad wheel bearings, damaged or rusted tone rings or bad sensors. It can even be a mismatch in tire size. It needs to be diagnosed by a knowledgeable technician.

Extremely common problem on GM trucks. This is called "false activation". Due to their design, the wheel speed sensors develop rather wimpy signals, and since relative speed between the sensor's coil and the disturbed magnetic field affects signal strength, at lower speeds the signal drops too low for the computer to be able to read it. It sees that as that wheel has locked up, so it tries to modulate brake fluid pressure in a misguided attempt at getting that wheel rotating again.

Another variable in the strength of the signal is the air gap between the tip of the sensor's magnet and the rotating toothed tone ring. The problem you're experiencing is caused by rust build-up under the sensor that pushes it outward, away from the tone ring. Add in the weaker signal as wheel speed slows down, and the signal drops too low before the truck comes to a stop.

You could solve the problem by replacing both front wheel bearing assemblies, but it is faster and just as effective to remove the sensors and scrape off the rust.

The first photo shows the bearing assembly as you'd pull it out of a box. The second one is of the sensor unbolted and pulled out. The blue arrow shows where the rust develops that pushes the sensor away from the tone ring. As long as it is removed, clean off any rust or metal filings that are clinging to the magnet at the red arrow. That can also lower the strength of the signal.

If you want to do this yourself, start with this article, but it's the video at the end that pertains to this service. You'll see how to start the process of removing the wheel bearing assembly, but you don't have to go that far. Use this to see how to get down to the wheel speed sensor.

Even better, check out this video:

https://youtu.be/ZgiPRG6jffc

Hi,

More than likely you have speed sensor that is not reading properly. So when you slow down it is telling the ABS that one is moving faster or slower than the others and the ABS thinks is it slipping and activates the ABS event.

We need to check for codes and then monitor the wheel speed and vehicle speed sensor and look for which is different. You will need a scan tool to do this. If you don't have one, just have a parts store check for codes and we can go from there. Thanks

This is a real common occurrence with GM trucks. It's called "false activation", meaning the ABS is kicking in when it isn't needed. Most commonly this is caused by rust build-up under the front wheel speed sensors. These sensors develop real wimpy signals to start with, then, as the rust pushes the sensor away from the tone ring, the signal gets even weaker, to the point the computer can't see it, and it interprets that as that wheel is locking up.

When a voltage is developed mechanically, you need a coil of wire, a magnet, and most importantly, movement between them. As the movement, or speed, slows down, the strength of the voltage gets weaker. That's why this problem occurs at relatively low speeds.

Start by removing the front sensors and looking for rust under them and on the bearing housing. My nifty red arrow in the photo is pointing to where that rust builds up.

Sounds great. Thanks CARADIODOC, I bet you are exactly right.

Let us know what you find. Thanks

Wheel speed sesnor are the sensors at each wheel. They are a magnet and they detect how fast the wheels are turning by measuring the gaps in a moving tone ring. That tells the computer how fast each wheel is turning and if one is locking up. They are easy to find. Look for the wires goign to each wheel what the attach to is the sensor

I hope so. I only know three things, and that is one of them!