GM Full-Size Trucks 1970-1979 Repair Guide

Bleeding the Brake System

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See Figure 1



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Fig. Fig. 1: When bleeding the brakes, you must keep the tube submeged in brake fluid

The brake system must be bled when any brake line is disconnected or there is air in the system.

Never bleed a wheel cylinder when a drum is removed.

  1. Clean the master cylinder of excess dirt and remove the cylinder cover and the diaphragm.
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  3. Fill the master cylinder to the proper level. Check the fluid level periodically during the bleeding process, and replenish it as necessary. Do not allow the master cylinder to run dry, or you will have to start over.
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  5. Before opening any of the bleeder screws, you may want to give each one a shot of penetrating solvent. This reduces the possibility of breakage when they are unscrewed.
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  7. Attach a length of vinyl hose to the bleeder screw of the brake to be bled. Insert the other end of the hose into a clear jar half full of brake fluid, so that the end of the hose is beneath the level of fluid. The correct sequence for bleeding is to work from the brake farthest from the master cylinder to the one closest; right rear, left rear, right front, left front.
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  9. The combination valve (all vehicles with disc brakes) must be held open during the bleeding process. A clip, tape, or other similar tool (or an assistant) will hold the metering pin in.
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  11. With power brakes, depress and release the brake pedal three or four times to exhaust any residual vacuum.
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  13. Have an assistant push down on the brake pedal. Open the bleeder valve slightly. As the pedal reaches the end of its travel, close the bleeder screw.
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Repeat this process until no air bubbles are visible in the expelled fluid.

Make sure your assistant presses the brake pedal to the floor slowly. Pressing too fast will cause air bubbles to form in the fluid.

  1. Repeat this procedure at each of the brakes. Remember to check the master cylinder level occasionally. Use only fresh fluid to refill the master cylinder, not the stuff bled from the system.
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  3. When the bleeding process is complete, refill the master cylinder, install its cover and diaphragm, and discard the fluid bled from the brake system.
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