Help - Weird Brake Issue - Unsolicited Rear Brake Lock Up

[m002922]

1998 k1500 z71. As the title says - rear brakes are locking up occasionally (it's done it when the vehicle is cold, and warm). This isn't I'm applying the brakes and they're locking up - it's I'm driving along at slow speed - generally just after coming to a stop and pulling away - never on deceleration - and they're locking up (no braking).

The last time this happened I pulled the ABS Circuit Breaker (got the ABS light) and it did it again 2x in a row within the span of 100'. The left rear brake was hot to the touch. At that point I had the vehicle towed to the shop.

The shop could not duplicate the problem - weren't seeing any codes in the ABS system. They checked the rear brakes and noted that the left was slightly tight, and loosened the shoes - but didn't see any setup issue. A week prior I had it in another shop that couldn't duplicate the problem and they also didn't notice anything unusual about the rear brakes.

I've heard proportioning valve - but this does it with no input from the pedal. I think I've ruled out the ABS (which I was having issues with, but new front hubs seem to have taken care of that).

I read a post somewhere about there being rear brake shoes out there that aren't designed properly - material that doesn't cover enough area of the shoe, and because of that limited area they can "catch" causing the brakes to lock. Anyone have any experience with that or can give me an idea of what I should be looking for?

I'm out of ideas and it scares me to drive the truck like this. All help appreciated.

 

[east302]

I wonder if the short section of brake hose above the rear axle is damaged, allowing flow to the cylinders but acting like a check valve when the pedal is released.

I suppose one way to check it is pull over as soon as they lock up and crack open a bleeder to see if fluid shoots out. I'm not really sure how you'd test it.

I guess that the shops looked at the hardware and didn't see any issues...no really deep grooves in the backing plates that would cause a shoe to stick?


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[m002922]

Both shops have looked at the hardware (cylinders, drums, pads, etc) and did not identify issues.

I've read about this suggestion as well - let's call it the "bad brake line / check valve theory." I'm starting to think this may be it, as when I had the truck in previously, the rear cylinders where completely dry and not moving - which would seem to indicate something creating an issue with getting fluid back there (though I'm not sure how you wouldn't catch that issue prior to things getting that bad) . . . maybe the check valve issue caused that other issue.

I just had a similar issue with another vehicle where a rubber rear brake hose collapsed, preventing fluid from getting back to the rear of the system (I'm not sure if this truck has one line running back that splits, or two separate lines created earlier). Maybe something similar is going on here.

I wonder if the short section of brake hose above the rear axle is damaged, allowing flow to the cylinders but acting like a check valve when the pedal is released.

I suppose one way to check it is pull over as soon as they lock up and crack open a bleeder to see if fluid shoots out. I'm not really sure how you'd test it.

I guess that the shops looked at the hardware and didn't see any issues...no really deep grooves in the backing plates that would cause a shoe to stick?


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[m002922]

FOUND THE PROBLEM!!!

So I had the rear brake hose that splits and goes to the two rear wheel cylinders replaced. That seemed to solve the problem . . . but then when I was 20 miles away from home it started again. Again it was after the car was warm, and it happened with no brake input . . . just starting from a stop and the rear left brake seemed to engage, locking up.

This time when it happened I could tell that the left wheel drum was definitely hotter than the right - so I was pretty sure the problem was on the left side only - and I was pretty sure that ruled out an issue with either the proportioning valve or the ABS system (as an issue with either of those would likely have been realized by both rear wheels).

I ended up pulling to the side of the road and taking off the tire and backing in the wheel adjuster all the way to limp it to the shop (because I can't do my own work because all my stuff is in storage).

With the new information I just told them to replace everything in the rear - shoes, drums, wheel cylinders, hardware, etc.

Turns out that it took removing everything from to see the issue. THE BRAKE BACKING PLATE. Even with everything off, it wasn't obvious. The mechanic who was working on it had seen the truck once before and not found the issue - and was pretty curious about the issue, so he spent a lot of time looking at the left side as he took it apart, and then compared it to the right. With everything off, and comparing the two, he could see the issue . . . but he said it wasn't obvious, and looking at the pictures I would agree.

Here is what a brand new aftermarket (not AC Delco) backing plate looks like. I'm wondering if these have been replaced once before on my truck - because mine look just like the aftermarket Doorman ones that come in pairs - which look different than the AC Delco Versions. These are the aftermarket Doorman versions:

https://drive.google.com/open?id=0B-m9TIIVqP6yYWJYTFJMZG9zTDA

https://drive.google.com/open?id=0B-m9TIIVqP6yMU5QUnVGSlNBNlU

Here is what mine looked like:

https://drive.google.com/open?id=1t_bnHaR5k3-1HjfwwxazuxTTMj0a7txnyA

https://drive.google.com/open?id=1343XcKFxCytHg3FeaZfHmMIa-1tGH1h2OQ

If you can see the difference, it looks like either the rivet corroded, giving way and allowing the brake shoe to move and catch, or the brake started catching (for some reason - which I'm guessing normally wouldn't be an issue if the rivet was strong) and then the rivet gave way. But you can see that the rivet is pulled more toward the front of the backing plate, putting the brake retaining mechanism - and therefore the brake in an "off" position.

Further, you can see that all the unsolicited breaking incidents (probably between 10 and 20 by the time the repair was done - and who knows how many at high speed that just weren't able to lock up the wheel) led to some bending in the shoe that was tucked into that part of the brake backing plate - as well as some very slightly odd drum wear.

https://drive.google.com/open?id=1PylRyWHSzxTFNAA0CSyu6ZiM2kc3i4NHWw

https://drive.google.com/open?id=1G1YPXpqKTCt_Ngz1da5XQRYEC5kei7IcCw

https://drive.google.com/open?id=1K4q7qbvHmzTRszwiwdLPVmTX8vwguGvttQ

So this is how the timeline played out.

I read way back at the beginning of this issue - "it's something with your rear brakes" - likely not an ABS issue. But the problem was my brakes weren't in great shape to begin with - I was having some intermittent ABS issues anyway as a result of noisy wheel bearings - so it took eliminating those (all new front hubs and front brakes) to narrow it down to it not being an ABS issue. When I had that done I had two separate shops check out my rear brakes and they couldn't get the truck to repeat the issue (just bad luck - because sometimes with me it would happen cold - and sometimes it would happen warm - but at the end it was generally happening once warm), and they said the rear brakes looked fine - which, in fairness, they did with everything assembled.

Then it happened again. When it happened again I pulled the ABS fuse, and it happened immediately again . . . so that ruled out the abs. Then, after I knew to be paying attention for it I figured out that it was happening completely unsolicited (it happened without applying the brakes) - which seemed to take the proportioning valve out of the equation. Then, out of ideas (because the rear brakes looked fine) I had them replace the rubber rear brake line . . . so that took that out of the equation. When it happened again (and that time I had the sense to check, and the heat of the drum was so much worse on the left), I was pretty confident it was associated with something with that side . . . so I just said replace everything in the rear brakes, and in the process of doing that the issue was found.

I'm hoping that this long winded thread will help someone that might have a similar issue . . . because even if your drums and drum brakes and hardware all look good . . . it might be something you can't really see (the backing plate). I had two shops that have very good reputations and are very familiar with these trucks look at it - and neither of them spotted it.

So there you go. I'm much poorer in having to track this down (because I couldn't do my own work), but, realistically, most of it needed to be done anyway. I'm hoping I can now have many trouble free miles . . . fingers crossed.

 

[m002922]

I just noticed there is an axle and brakes section where this should have gone . . . guess the heading threw me off.

If an admin wants to move it over there, feel free. Sorry for any confusion.

 

[east302]

Thanks for the update, glad you got it sorted out.


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