Virtually all modern vehicles 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 on the front of the master cylinder and assists the driver in applying the brakes, reducing both the effort and travel he must put into moving the brake pedal.
The vacuum diaphragm housing is normally 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, braking 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 car and that the pedal feels harder than usual.
Vacuum Leak Test
- Operate the engine at idle without touching the brake pedal for at least one minute.
- Turn off the engine and wait one minute.
- Test for the presence of assist vacuum by depressing the brake pedal and releasing it several times. If vacuum is present in the system, light application will produce less and less pedal travel. If there is no vacuum, air is leaking into the system.
- With the engine OFF , pump the brake pedal until the supply vacuum is entirely gone.
- Put light, steady pressure on the brake pedal.
- Start the engine and let it idle. If the system is operating correctly, the brake pedal should fall toward the floor if constant pressure is maintained.
Power brake systems may be tested for hydraulic leaks just as ordinary systems are tested.
REMOVAL & INSTALLATION
See Figures 1 and 2
Make sure all vacuum lines and connectors are in good condition. A small vacuum leak will cause a big problem in the power brake system.
- Disconnect the negative battery cable.
- Remove the master cylinder from the brake booster with the brake lines still connected.
- Disconnect the vacuum lines from the booster.
- Disconnect the booster pushrod at the pedal clevis.
- Remove the booster mounting nuts from under the dashboard.
- Lift the booster from the engine compartment.
- On vehicles with an adjustable pushrod, adjust the brake booster rod so the measurement between the booster and the centerline of the clevis hole is 4.92 in. (15mm).
- Install the brake booster assembly and tighten the attaching nuts to 9-1 ft. lbs. (2-16 Nm).
- Adjust the master cylinder pushrod and install the master cylinder assembly.
- Connect the booster pushrod to the pedal clevis.
- Connect the vacuum lines to the brake booster.
- Connect the negative battery cable.
- Start the engine and check for proper brake operation.