Question about 2004 GMC Sierra 2500

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Brakes locking up

All four brakes locked up on my 2004 gmc sierra, upon further inspection i noticed that the wires to my front left abs sensor was cut. I fixed the wires drove about 15 miles and they locked up again. I then pulled the fuse to the abs, let the rotors cool down again, and drove another 10 miles with no problems. I reinstalled the abs fuse and drove the remaining 15 miles home with no issues. I'm assuming i have an issue with the master cylnder, but i was wondering if anybody has had similar issues or knows of a possible solution. Thanks in advance for all the help.

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  • karleegourle Jun 02, 2009

    Thanks logbow06, good info, since the original post I did bleed the entire system, no luck. However I now ruled out all but two items. The master cylnder and the hydroboost. After doing more extensive reaserch on the hydroboost (and how it actually funtions) I have decided to replace both the hydroboost, to include flushing and refilling the powersteering fluid, and the master cylnder. This should take care of all issues. Thanks again for the info.

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Similar problem on a Toyota truck. Turned out to be a combination of items.

Bad valve in the master cylinder.

Contaminated brake fluid.

Basically the heat in the rotors was boiling the brake fluid cause it to expand and lock up the brakes.

Not sure if the fluid contamination was caused by the master cylinder or the other way around. (it is the whole chicken or the egg argument.

My recommendation is start at the right rear wheel. Drain the fluid until it is at the bottom of the reservoir. refill with new fluid. Bleed the entire system. working your way around the truck from longest run to shortest. (Distance from master to the wheel)

Take your time and don't rush the job. After you have flushed and bled the system, go for a check ride. If the condition continues, you likely have the defective seal in the master cylinder.

Good luck, let me know if you need any other info.

Posted on Jun 01, 2009

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