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Tim West
Tim West, ASE Certified Technician
Category: Car
Satisfied Customers: 4638
Experience:  ASE Certified Service Manager for Bridgstone/ Firestone
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1979 GMC: wheel cylinders..master cylinder due to brakes locking up

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I have a 1979 GMC 7000 series truck. I have replaced all wheel cylinders and master cylinder due to brakes locking up. I was told that I would need to replace these parts. The brakes will not work now. What else do I need to do? Thanks in advance for your advise.
What do the brakes do? when does the problem start? what happens?
Customer: replied 9 years ago.
The brakes do not work at all. It appears the the new master cylinder is defective. I ordered a replacement master cylinder to see if that's the problem. As I apply/pump the brake pedal to build up pressure, the brake pedal remains soft and goes all the way down. I have also bled the brake system.
How did you bleed the system? HAve checked for leaks everwhere
Customer: replied 9 years ago.
I was told to bleed the system from the master cylinder. There are no leaks.
Bleed from the master cylinder? Did you bench bleed the master before installing it?

Many times the correct bench bleeding of the master cylinder is an overlooked task. However, this is a MUST and should be performed any time a new master cylinder is installed. If the master cylinder is not bled, it will take you at least twice as long to bleed the system, and then there's no guarantee that you'll have removed all the air from the system.

Before bench bleeding the master, completely install all of your replacement or upgraded brake components, including the master cylinder and brake lines; this is so you can hook up the master cylinder as soon as possible after it has been bench bled, to keep gravity from leeching fluid from the master cylinder during the installation of the other components. After installing the system, remove the master cylinder completely from the vehicle. Master cylinders MUST be bench bled outside the vehicle and without any other components attached. This means that if a new master is equipped with a booster, proportioning valve and plumb lines attached, they must all be removed prior to bench bleeding.

Be sure to place the master cylinder level in a vice and secure it properly. If the cylinder isn't level, not all the air will be able to be removed from the master cylinder. Remove the master cylinder top and fill with fresh brake fluid to the appropriate level as indicated in the master cylinder-- generally about 1/2 to 1 inch from the top of the reservoir.

Insert a pair of short, looped brake lines or rubber tubing from a bench bleeding kit, (available at nearly all part stores), into the two side ports of the master cylinder. (Note: on dual-ported master cylinders where there are four ports, choose one side and use both ports on that one side to perform the bleed process.) Insert one rubber tube into each of the plastic fittings. and the loose ends should be inserted into the master cylinder reservoir. The plastic tab should be used to hold the tubes in place by slipping it over the reservoir separator and the hoses through the round holes.

Using a wooden dowel, a blunt metal rod, or a large Philips screwdriver, compress the master cylinder plunger with slow, deep strokes. Once the large bubbles have subsided, it should become increasingly more difficult to compress the master cylinder piston. Continue to do so with slow, short strokes at the bottom of the piston stroke until no more bubbles appear. The rubber hoses and plastic fittings may be removed from the master cylinder. Secure the master cylinder top, install the master cylinder and leave enough room on the mounting nuts to allow for some movement of the master. This will allow you some breathing room while screwing in the hydraulic lines to help prevent cross-threading the fittings.

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Customer: replied 9 years ago.
Yes , I bench bled the master cylinder but did not use the a bleeding kit method you described. I think that's where I went wrong. I am going to use this method on the new master cylinder and let you know the results.
Customer: replied 9 years ago.
Problem Solved! Thanks Tim.
sweet, take care thanks for choosing JA