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Brakes locking up for no reason

sahls01gmc

By sahls01gmc
in Ask A GM Technician

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sahls01gmc Senior Enthusiast

sahls01gmc

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I have an 01 GMC Sierra 2500 (non HD) with 232,000 miles. I've been having problems with the brakes lately. Recently, the brakes would not release after you let off the pedal, and continually got worse until you could not physically move the truck. After about 3 to 4 hours, everything had "cooled off" enough and the truck would drive as normal. I took it to a shop, and they said that the front calipers were done, and needed to be replaced. I replaced them, and had the system flushed. The same thing has just happened this morning. I had been driving on the highway for 15 miles, and I didn't even touch the brakes. Gradually, I could feel the truck slowing down unless I gave it more gas. By the time I got off the highway and parked off the road, the rear brakes were smoking. And the brake pedal was as hard as a rock. I could smell the front brakes as well, but they were not smoking.

 

The 2 Brake lights that are on the dash have lit up from time to time, but i have not been able to check the codes, and they only sporadically come on.

 

What problems should I be expecting? I have recently replaced both wheel bearings, so I think I can rule those out at this point.

 

Thanks in advance for any help that you can offer.

 

 

James

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Doug_Scott
Doug_Scott

I agree that it is likely in the master cylinder system. By that I mean it is likely somewhere between the rubber on the brake pedal and the brake lines that attach to the master cylinder. Could eve

jamiecd77
jamiecd77

I have had the same problem, and after 3 months of trying to figure it out,we finally disconnected the master cylinder from the brake booster. The front brakes that we're locking up instantly released

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Doug_Scott Senior Enthusiast

Doug_Scott

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Posted

Does the truck have rear disc or drum brakes? If drum brakes, then the compensating port in master cylinder is blocked. If disc, then you need to look at the flex hoses to see if they are breaking down internally creating little flapper valves that restrict the brake fluid from flowing back when brakes are released. Try opening the bleeder screw when the wheels are locked up again and see if releasing the fluid releases the brakes. If it does, then get the brakes to lock up again, but this time loosen the brake line at the master for the rear brakes and see if the brakes release instantly. If they don't, then you need to move down the line to the next component and loosen the line there. Keep doing this until you get the brakes to release. The part that is directly upstream from the connection you loosened is where the problem is.

 

If drum brakes, loosen the line at master cyl and see if it releases the brakes. If it does, then it is either the master cylinder or the brake rod from the pedal to booster, or the rod between the booster and master being too long for some reason.

sahls01gmc Senior Enthusiast

sahls01gmc

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Thanks Doug. It does have disc brakes in the rear. I noticed that all 4 brakes were seeming to close. It never pulled from one side to the other. Should I still assume that this is a flex line? I believe I will start looking at the Master Cylinder lines first.

 

I'm a lil nervous messing with the brake lines, as they are the only thing that stops me.

 

 

Thanks for the info. I'll update as I go along.

Babyhauler Enthusiast

Babyhauler

Posted
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Quote by sahls01gmc

 

I'm a lil nervous messing with the brake lines, as they are the only thing that stops me.

 

That's a great line. So true.

 

I was initially thinking sticking caliper pins, but they should not cause all of those issues. The brake lines degrading sounds much more reasonable.

Post how you solve it.

 

Good luck,

 

Babyhauler

sahls01gmc Senior Enthusiast

sahls01gmc

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I was just thinking, how likely is that the brake lines are going out when I drove 15 miles without even touching the brakes? I was cruising at 70mph, and then all of a sudden they started slowly locking up, and this time the rears worse than the fronts.

 

I'm an electrician, so I completely understand the logic of something not likely being the actual cause of the problem. I just can't wrap my head around a brake line failing when they were not even in use at the time that it happened.

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
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Brake lines start to break down internally, and the innermost "tube" starts to flake. The flaking becomes little flapper valves that will allow fluid to flow in one direction, but not in the other direction. It is virtually impossible to start your truck, and drive it to the highway without using the brakes at all. Not sure when they started, but don't you need to apply the brakes to get it out of park? If it is all 4 wheels doing this, it will not be flex lines. It will most likely be something in master or booster or something holding the brake pedal down slightly or the brakes applied slightly. Since it is all 4 wheels, it actually makes it far easier to find. You need only look at the parts common to all 4 wheels.

sahls01gmc Senior Enthusiast

sahls01gmc

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Posted (edited)

That is true Doug. I had used the brakes to turn on a street, and onramp. I guess it could be possible that although I could still move forward, there was enough friction on the pads and rotors to eventually heat up enough to stop the truck.

 

With all 4 tending to lock up, even though the rear was worse this time, I would believe to start checking the master cylinder. Would this be a correct assumption based on the fact that when they lock up, the pedal is hard as a rock?

 

Thanks again

Edited by sahls01gmc
00Silverado4x4 Senior Enthusiast

00Silverado4x4

Posted
Posted

I'm just thinking along with all others suggestions, might want to do complete fluid flush/change as well. That's a lot of miles and the fluid may be shot.

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

Master cylinder would be the place I'd start. Seems like that's the only way it'd hold pressure on all 4.

 

I agree that it is likely in the master cylinder system. By that I mean it is likely somewhere between the rubber on the brake pedal and the brake lines that attach to the master cylinder. Could even be as simple as a brake light switch causing the brake pedal to not return to the fully released position. Since the actual issue is so severe, it should actually be easy to find the cause.

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sahls01gmc Senior Enthusiast

sahls01gmc

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I believe I solved the mystery. The brakes locked up, and I went to the Master Cylinder and loosened the brake line that was closer to the front of the truck (which I believe is the rear brakes). I loosened it enough for fluid to come out, and then all of a sudden the wheels broke free and the truck "rocked" into park.

 

 

I will confirm this hopefully tonight if I can get it replaced.

 

 

Thanks everyone for the help. Now let's hope I bench bleed the master cylinder correctly so that the brakes still work next time I need them!

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

Before you replace the master cylinder, get the brakes to lock up again, but this time loosen the master cylinder from the booster. Back both mounting nuts off enough to make the master cylinder very loose. If the brakes release, it will be something before the master cylinder, like brake pedal not returning enough, or the rod inside the booster.

sahls01gmc Senior Enthusiast

sahls01gmc

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I have replaced the master cylinder and was able to drive the truck for 20 minutes and not even a thought of locking up. Doug, I will keep an eye for that possibility.

 

Let me know if this scenario is normal. Keep in mind this is before I replaced the master cylinder.

 

When I go to start the truck, I have my foot slightly on the brake pedal. When I start it, the brake pedal almost seems to "pop" back against my foot. Is this normal? I understand that the initial lack of pressure is then pressurized once the engine is running. Not sure if that helps narrow the possibilities of failure.

 

 

Thanks a bunch to everyone for their assistance. I will keep you posted if something else decides to turn up.

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

I have replaced the master cylinder and was able to drive the truck for 20 minutes and not even a thought of locking up. Doug, I will keep an eye for that possibility.

 

Let me know if this scenario is normal. Keep in mind this is before I replaced the master cylinder.

 

When I go to start the truck, I have my foot slightly on the brake pedal. When I start it, the brake pedal almost seems to "pop" back against my foot. Is this normal? I understand that the initial lack of pressure is then pressurized once the engine is running. Not sure if that helps narrow the possibilities of failure.

 

 

Thanks a bunch to everyone for their assistance. I will keep you posted if something else decides to turn up.

 

 

The pedal moving when the truck starts is completely normal. You have the reason backwards though. The booster does not ever have a positive pressure in it. After not running for a bit, the vacuum in the booster will go away, leaving the front half of the booster at the same air pressure as the rear half of the booster. Once the truck starts the pressure will drop to a negative reading on a pressure gauge, or a positive reading on a vacuum gauge. The booster uses the pressure differences between the two halves of the booster to help "pull" the pedal down.

  • 1 year later...
jamiecd77 New Member

jamiecd77

Posted
Posted (edited)

I have had the same problem, and after 3 months of trying to figure it out,we finally disconnected the master cylinder from the brake booster. The front brakes that we're locking up instantly released. I have searched high and low on the internet for a solution to this problem and I am pretty sure this is finally it. if the new booster we are about to put in does not fix it I will post again. I have replaced the ABS modulator valve, brake hoses,, calipers, pads, master cylinder, and rusty brake lines. Wish I had found this forum months ago...

Edited by jamiecd77
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