Brakes lock up

Tiny
JONATHAN MOORE2
  • MEMBER
  • 2002 CHEVROLET TRAILBLAZER
  • 4.2L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 130,000 MILES
Brakes lock up after fifteen to twenty minutes of driving to the point of completely stopping me if I take my foot off the gas. After another fifteen to twenty minutes they are back right and the cycle continues. I can release the line on my master cylinder and the brakes will release, my master cylinder push rod is not adjustable. Brakes were bled previous by machine/scanner, 100% new fluid, brake pads and rotors are all in OEM tolerances, lines, ABS module, master cylinder all new within a six month range. No vacuum leaks as far as I can tell. I have exhausted everything I can think of. Can somebody help please?
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Monday, May 20th, 2019 AT 7:43 PM

3 Replies

Tiny
CARADIODOC
  • EXPERT
If loosening the lines at the master cylinder let the brakes release, as you observed, there's two things to look for. One is the brake light switch is over-adjusted and is holding the brake pedal down a little. You can identify that by loosening the master cylinder's mounting nuts and letting it pull away from the booster 1/8" or so.

The more serious cause is when the brake fluid has been contaminated with a petroleum product. Running new fluid through the system won't solve that. The rubber seals in the master cylinder will grow past the fluid return ports and keep the fluid trapped when pressure builds up as the brakes get warm. The more the truck is driven, the harder the brakes will self-apply. One way to identify this is to observe the rubber bladder seal under the reservoir cap is blown up and mushy, and will be hard to press back into the cap.

The only proper repair for contaminated brake fluid is to remove all parts that contain rubber that contacts the fluid, flush and dry the steel lines, then install all new parts. That includes calipers and wheel cylinders, rubber flex hoses, combination valve, rear height-sensing proportioning valve when used, master cylinder, and ABS hydraulic controller. If any part is not replaced, including those that were replaced six months ago, the contamination will leach out of it and recontaminate the new fluid and parts.

Being a mechanic, you're familiar with a bleeder ball for bleeding brakes. An inexperienced mechanic at a mass merchandiser's auto shop used a funnel used for engine oil to fill the bleeder ball at his shop in the 1980s. He conscientiously wiped it out first with a rag, but no one ever explained to him how the residue was plenty to contaminate the brake fluid. He put over four gallons of fluid in the ball, then used it for the next brake job. That shop ended up doing thousands of dollars worth of repairs on over a dozen cars once they realized what had happened. We even used to run into this with older rear-wheel-drive cars when we used our hands to repack the front wheel bearings. Often we'd wipe our hands on a rag, then use a fingertip to poke the rubber seal back into the reservoir cap when we filled the brake fluid. The grease residue where we touched the seal was enough to contaminate the brake fluid.

I mentioned those horror stories to stress how little contamination can cause a big problem. I hope that is not what you find.
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Tuesday, May 21st, 2019 AT 4:25 PM
Tiny
JONATHAN MOORE2
  • MEMBER
Thank you for the quick response, I honestly don't believe contamination is the issue, it did not have it prior and cap never looked mushy always looked good. The reason for the fluid exchange is this is a completely custom truck brakes and lines were disconnected blown through and at first we were having an issue bleeding calipers (ended up being bleeder valve wasn't sealing, we fixed it) but we ended up with new fluid throughout the system just from how many times we bled.(Manually, with a scanner and with the brake vacuum using an air compressor) I actually thought of running a few spacers to see if that would make a difference I may try that tomorrow. I did readjust the pedal but no changes. I doubled checked the vacuum line going to the booster for cracks, and also checked the check valve on the booster, it was good but I still replaced with a new part. Have you ever heard of a brake booster causing issues as I have described, I had a few guys tell me it was, but I haven't run into it, we don't really focus on brakes this is just a really good customer which I don't want to let down or have him throw money at a solution that doesn't work. I appreciate any other input you might have.
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Tuesday, May 21st, 2019 AT 7:11 PM
Tiny
CARADIODOC
  • EXPERT
The options are really limited because if loosening the lines at the master cylinder releases the brakes, that is where the fluid has to be trapped. If you're doing that with the engine not running, the power booster is not in the picture, but regardless, while we do think about it, I've never actually had a vacuum booster cause the brakes to self-apply.
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Wednesday, May 22nd, 2019 AT 3:12 PM

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