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Clayso, Master Automobile Technician
Category: Chevy
Satisfied Customers: 880
Experience:  ASE Master Certified Automobile Technician. 30 Years Experience.
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Chevrolet Avalanche 1500 Z71: I have a 2004 chevy avalance

Customer Question

I have a 2004 chevy avalance all four brakes lock up .they have change everything but master cyclinder and fluid and still locks up all four wheels.
Submitted: 2 years ago.
Category: Chevy
Expert:  Clayso replied 2 years ago.
Hi There, I'm Clayso a certified master automobile technician with 30 years of automotive repair and diagnostic experience and I'm here to help with this problem. Additional services may be offered for detailed information.
From your description it sounds like the Hydraulic Assist/hydro boost assembly is continuing to apply pressure when the brakes are released. When the brakes are cool and not locked up. Start and run the engine to operating temperature. Hold the engine at about 2000 rpm. Turn and hold the steering wheel all the to the left or right for a couple of seconds. Return the vehicle to idle, put it in gear and see if the brakes are now locked up. If the brakes are not locked up the vehicle will have to be driven until the brakes do lockup.
When the brakes are locked up loosen the two nuts that hold the master cylinder to the booster a couple of turns each, if the brakes release the hydraulic booster is at fault. If the brakes do not release with the master cylinder loose from the booster loosen the brake lines at the master cylinder. If the brakes are now free the master cylinder is maintaining the pressure and is at fault.
Brake Assist System Description and Operation (Hydraulic Assist)
System Component Description
The brake assist system consists of the following:
Brake Pedal: Receives, multiplies and transfers brake system input force from driver.
Brake Pedal Push rod: Transfers multiplied input force received from brake pedal to brake booster.
Hydraulic Brake Booster: Uses power steering fluid pressure to decrease effort required by driver when applying brake system input force. When brake system input force is applied, pressurized power steering fluid is admitted to the rear of an apply piston, providing a decrease in brake pedal effort required. When input force is removed, pressurized power steering fluid is blocked off from the piston.
Power Steering System: Supplies pressurized power steering fluid used by hydraulic brake booster to decrease brake pedal effort.
System Operation: Brake system input force is multiplied by the brake pedal and transferred by the pedal push rod to the hydraulic brake master cylinder. Effort required to apply the brake system is reduced by the hydraulic brake booster.
I hope this information is helpful to you.
Should you have any questions please let me know.
Positive ratings are greatly appreciated.
Thanks so much.
Expert:  Clayso replied 2 years ago.
Hi There,
You last viewed this thread on June 22nd, and I hope the information I gave was helpful. Let me know if you have followed through with my recommendations. If you have any questions, please let me know.
Thanks so much.
Customer: replied 2 years ago.
THANK you the mechanic changed the master cylinder said that was the problem.
Expert:  Clayso replied 2 years ago.
You're welcome. I'm glad the problem is solved. If it's not to much to ask would you rate my answer in a positive manner.
Thanks so much.