I recently bought a 2005 GMC Z71 (5.3 4x4) to replace my '03 that got totaled. It has drums in the rear and everything seemed ok with the brakes, at fist. I had to do a quick panic stop the other day though and it locked up the rear brakes and they stayed locked even when I took my foot off the pedal. The brakes then stayed locked up and wouldn't let the truck move (without doing a huge burnout). The only way I finally got it to unlock was to put in in reverse and back into gear a few times and push and release the parking brake a few times, then it finally let them free. I tested it a few more times and realized that this wasn't an isolated incident, so

I took it to a local suspension and brake shop that I know well. They checked everything out, but said that the rear drums and all the brake hardware was all good. The truck does in fact stop good, but the locking brakes is scary as hell. Anyway, he figured it was an ABS issue and we pulled the fuse and sure enough, we couldn't get it to do it again after that. He said that the only time he has seen one do something like this was when the ABS module under the truck was bad and he suggested that I go back up to the dealership and try and get them to repair this for safety reasons before I dump a bunch of my own money into it since I just got the truck about 10 days ago.

Has anyone else had this issue before? I found some threads about ABS issues from the factory, and even possible recalls, but I couldn't find anything specific about it locking the brakes up. Seems like the valves inside the ABS block are sticking and causing the fluid to get trapped and lock the brakes up.

Also, does anyone know if the dealership can be held responsible for fixing something like this since it's a major safety issue and I purchased the truck less than 30 days ago?

 

So I took the truck back up the the dealership today. They checked it out really quick and immediately said that the fluid was contaminated and that they would basically have to replace every seal in the braking system to make it safe to the tune of $3,000. I went and talked to the used car manager and he is trying to get me to trade it for another truck, but can't guarantee there won't be a price difference. The brakes work PERFECTLY without the ABS fuse and this is a very high optioned truck with an L33 engine and it's in otherwise immaculate condition. I don't know if they are trying to screw me here because they don't want to cover the repair or what! They didn't even drive the damn truck either to test out what I told them. They simply looked at the fluid and that was it. I really don't want to trade this truck if I can help it.

Has anyone else faced these same brake system issues? I have had a lot of cars, mostly sports cars, and usually have just changed out the brake fluid if it looks bad. I've never heard of having to replace the entire system just over contaminated fluid though, if that is even what's really causing the issues. Brake pedal feels good and firm and brakes grab hard. I wouldn't think it would do that if there was moisture or something in the system.

 

$3000?? LMFAO. Such crooked shysters.

The ABS systems on these trucks are known to be garbage, could be a dirty/faulty ABS sensor on the front wheels. Otherwise, I would just keep ABS disabled because it doesn't work well to begin with anyways.

 

That's the way I feel too. I am almost 100% certain it's just the ABS. Let's say the fluid is bad though.....could it really cause all that damage or should I just flush it and forget it.

 

General rule of thumb for brake fluid is to flush it every 2 years, outside of that a copper test should be used to confirm the copper content in PPM. looking at the fluid color is not an acceptable diagnoses, esp... for ABS lockup, ABS failure is no joke, and given the answer they gave you it should be clear to move on to somewhere else.

To me it sounds like the dealer failed to preform the brake flush correctly before selling you the truck, once you mashed the brake pedal the ABS was engaged and a possible air bubble is holding the pressure to the rear brakes, or a worn ABS valve in the valve block, would not release. I don't think its the master cylinder "replenishing valve" but its worth a look since it can hold brakes if the fluid cant return to reservoir. You didn't list any ABS codes, even then I doubt its a ABS wheel sensor.

There is a few GM techs on here maybe they can enlighten us on some known issues.

btw, I would not daily drive around without ABS.

 

It actually never gave me any ABS or brake codes (or at least one to trigger a light on the dash) at all, even when it locked up, but when I pulled the ABS fuse, it hasn't had that issue since. Stoops perfectly with no issues with the ABS fuse pulled, but obviously has the annoying codes on the dash all the time, which would mask any other codes that may come up. Put the fuse back in and the codes go away, but the locking problem happens again (only when you make ABS engage).

 

I had the same problem on my 03. I took out the abs fuse, since a couple of times I almost crashed because of the brakes locking up. Can not remember what I replaced that actually fixed the problem, but might have been replacing the hub. Sorry not much help.

 

sounds like a ABS unit, can you go lock up the vehicle in your driveway or street, with a quick sprint and then engage the ABS. Crack it from the master first if they let go then maybe a master, if not then work back to the ABS unit and crack the lines to the rears and see if they release, if not then also look at the rear drum cylinders. After you will need to bleed the brakes, so be ready for that as well.

 

So I did a bit of research and found that if a petroleum based fluid was ever accidentally added into the system, it can cause the rubber seals to soften and expend. I went out and looked at the master cylinder and the inside rubber seal of the cap is all swollen and cracked. I pulled a little fluid out and it has a bit of a dark green tint to it. I read to mix a little water into a sample of fluid and it should dissolve all the fluid into a milky solution. If there is any petroleum contaminants, they will float on top. It pretty much dissolved it completely, but there is a thin ring at the top outer edge where the solution meets the plastic cup. This may be normal, but hopefully it's not evidence of petroleum contaminents. I bet they just took that cap off and saw that the rubber was all swollen and that was evidence enough for them. I'm not really sure what my next move should be at this point, but like I said, the truck still stops fine and doesn't lock up as long as the ABS is disengaged.