See Figures 1, 2 and 3
The hydraulic brake system must be bled whenever a fluid line has been disconnected because air gets into the system. A leak in the system may sometimes be indicated by a spongy brake pedal. Air trapped in the system is compressible and does not permit the pressure applied to the brake pedal to be transmitted solidly through the brakes. The system must be absolutely free from air at all times. When bleeding brakes, bleed at the wheel most distant from the master cylinder first, the next most distant second, and so on. During the bleeding operation the master cylinder must be kept at least 3 / 4 full of brake fluid.
On 1974 and later models, there is a combination pressure differential (failure warning) and proportioning valve in the hydraulic system. It is in the engine compartment on the inner side of the left frame rail. When bleeding the brake system, the metering section of the valve must be held open. On 1974-76 models, remove the warning switch wire, switch terminal, plunger, and spring from the valve. On all 1977 models and 1978-79 models using a valve assembly with a flat exterior surface, remove the plastic dust cover at the end of the valve and hold the valve stem OUT. On 1978-79 models using a valve assembly with a rounded exterior surface, hold the valve stem IN by pressing against the dust cover boot. Tools are available to hold the valve stem in the proper location or can be fabricated.
To bleed the brakes, first carefully clean all dirt from around the master cylinder filler cap. If a bleeder tank is used follow the manufacturer's instructions. Remove the filler cap and fill the master cylinder to the lower edge of the filler neck. Clean off the bleeder connections at all four wheel cylinders. Attach the bleeder hose to the right rear wheel cylinder bleeder screw and place the end of the tube in a glass jar, submerged in brake fluid. Open the bleeder valve 1 / 2 - 3 / 4 of a turn. Have an assistant depress the brake pedal slowly and allow it to return. Continue this pumping action to force any air out of the system. When bubbles cease to appear at the end of the bleeder hose, close the bleeder valve and remove the hose.
Check the level of fluid in the master cylinder reservoir and replenish as necessary.
After the bleeding operation at each wheel cylinder has been completed, fill the master cylinder reservoir and replace the filler plug.
Do not reuse the fluid which has been removed from the lines through the bleeding process because it contains air bubbles and dirt.