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Brakes fail when power applied to them.

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Guest
Anonymous Poster
fagan@advbusdev.com

Aug 22, 2009, 5:57 PM

Post #1 of 1 (2552 views)
post icon Brakes fail when power applied to them. Sign In
I have a 1989 GMC Sierra 7.4L, two wheel drive w/air/PB/PS 1/2 ton, long bed truck. My brakes are failing. Weeks ago, the brake light came on and I switched out the Abs computer. This worked for a while intermittently, then the light came back on, stayed on and the power assist stopped working. By power assist, I mean the brakes were not pumped up like they were and went to the floor. Last time I took the truck out, the front brakes got hot, it felt like there was pressure building in the line and they would not release. They got hotter and hotter until they started to smoke. I was on the freeway when this happened. Chevy said it looked like the master cylinder was sticking and holding the pressure to the front brakes. After I got it home, I replaced the master cylinder and the power assist unit under the master as a whole unit.
I bled the brakes twice as they had air in the lines and when pressure is applied to them, they feel squishy. When I turn the car on and power is applied to the brakes, they go all the way to the floor the first time. When the car is on, I can pump the brakes up until they work about half way. The truck will stop but they feel squishy and the power assist does not seem to be working. I bled the power assist unit. I have checked the fuses under the dash and they are ok. I am fairly sure the abs computer is working as the light on the dash turned off as soon as I replaced the master cylinder and power assist unit. One more thing. When I pump the brakes, I can see fluid blowing back into the fluid resevior as if the power assist is not opening and letting the brake fluid through to the assist unit. A lot of blow back. I can not find any other fuses other than the ones under the steering wheel. What can I test next to determine what is not working.





 
 
 






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