Virtually all vehicles today use a vacuum assisted power brake system to multiply the braking force and reduce pedal effort. Since vacuum is always available when the engine is operating, the system is simple and efficient. A vacuum diaphragm is located between the master cylinder and the firewall and assists the driver in applying the brakes, reducing both the effort and travel that must be put into moving the brake pedal.
The vacuum diaphragm housing is connected to the intake manifold by a vacuum hose. A check valve is placed at the point where the hose enters the diaphragm housing, so that during periods of low manifold vacuum brake assist will not be lost.
Depressing the brake pedal closes off the vacuum source and allows atmospheric pressure to enter on one side of the diaphragm. This causes the master cylinder pistons to move and apply the brakes. When the brake pedal is released, vacuum is applied to both sides of the diaphragm and springs return the diaphragm and master cylinder pistons to the released position.
If the vacuum supply fails, the brake pedal rod will contact the end of the master cylinder actuator rod and the system will apply the brakes without any power assistance. The driver will notice that much higher pedal effort is needed to stop the vehicle and that the pedal feels "harder" than usual.
REMOVAL & INSTALLATION
See Figures 1 and 2
- Slide back the clip and disconnect the vacuum supply line at the brake booster. Pull the hose off gently in order to avoid damaging the check valve.
- Remove the master cylinder as described previously.
- Disconnect the pushrod at the brake pedal. Pull the cotter pin or lockpin out of the pedal clevis. Separate the rod from the pedal.
- Remove the mounting bolts and nuts from the firewall and remove the booster.
- On the 1983-85 diesel powered Pick-ups, and the 1979-86 gasoline powered models, remove the two gaskets and the booster spacer from the firewall.
- Install the booster and tighten the nuts and bolts to 9 ft. lbs. (12 Nm). Do not overtighten these bolts.
Some boosters use a spacer block and gaskets between the firewall and the booster. If present, they must be reinstalled in the correct order and position.
- Connect the brake pedal to the pushrod and install a new cotter pin. A light coat of multipurpose grease on the clevis pin and washer is recommended.
- Before reinstalling the master cylinder, the booster pushrod clearance must be calculated and adjusted. This is particularly important if the booster has been replaced. Please refer to the master cylinder procedure given previously and perform the necessary steps.
- Install the master cylinder and bleed the brake system completely.