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john 7/13/2016

2007 GMC Sierra 1500 Classic SL 6 Cyl 4.30L


Why are my front brakes locking up when temp reaches 90 degrees or higher

driving to work at 5 am, there are no problems with the brakes but on the way home in the afternoon, after the temp reaches 90 degrees, the front brakes start to apply without my help until it almost stops the truck. I then must stop the truck, open the hood, let the vehicle cool off for 45 minutes and I actually hear the calipers slowly release their grip on the rotors. also, today was very hot so when I finally started it up again, now the abs light is on. Help please. By the way, I have disc fronts and drum rears.

1 Answer


Jimm 7/13/2016

The front hydraulic flexible brake lines start to break down internally, and the innermost "tube" starts to flake. The flaking material becomes little flapper valves that will allow fluid to flow in one direction, but not in the other direction.

Replace both the front brake caliper flexible hydraulic hoses, along with new brake fluid in the system.

One additional test - when the brakes lock up, and at the master cylinder - loosen the brake line that is closer to the front of the truck (which I believe is the rear brakes). Loosen it enough for fluid to come out - see what happens.
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.

Try these on-line resources for the replacement parts: www.autopartswarehouse.com, www.partsgeek.com, www.rockauto.com, www.partstrain.com - to list a few.

For example, they (RockAuto) list the replacement front hydraulic hose from around $10.25, and the brake master cylinder from around $39.00 for your vehicle.

Front left brake hose = RAYBESTOS part #BH383006 - or it's equivalent
Front right brake hose = RAYBESTOS part #BH383007 - or it's equivalent

Brake master cylinder (with reservoir) = AC DELCO part #18M2440 - or it's equivalent


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