Repair Guide: Power-Assisted Brakes

A power-assist brake system is used on most cars to reduce the braking effort required by the driver. There are two general types of power-assist brakes. The most common type uses an intake manifold vacuum acting on a diaphragm to provide the power assist. The other type uses hydraulic power developed by a pump to provide the assist. The vacuum power-assist brake uses a large diaphragm in a canister behind the master cylinder. The canister is connected by a hose to the engine intake manifold. The push rod connected from the brake pedal goes through the power-assist unit on its way to the master cylinder. Inside the canister is a large diaphragm. This diaphragm is connected to the push rod. Engine intake manifold vacuum is applied to both sides of the diaphragm. When the brakes are applied, valving inside the canister is operated by the push rod. Atmospheric pressure is admitted to one side of the diaphragm. With vacuum on one side and atmospheric pressure on the other, the diaphragm moves toward the master cylinder. Because the push rod is connected to the diaphragm, it also moves. The large diaphragm has enough force to take much of the effort of applying the brakes away from the driver.

Hydraulic boosters require a hydraulic pump to provide the power assist. In many systems, the power steering pump is used to provide power for both the power steering and power brakes. As shown below, fluid under pressure goes from the power steering pump to a hydraulic boost unit behind the master cylinder. This unit contains a power piston and valving. When the brakes are applied, hydraulic pressure pushes on the boost piston, which in turn pushes on the brake push rod. The hydraulic power takes much of the effort out of brake application.

The hydraulic booster normally has an accumulator, which is a device that is spring loaded or charged with pressurized gas. This device provides emergency pressure to apply the brakes should the power steering flow be interrupted for any reason. The system is good for one to three stops, depending on the type of accumulator. Dash warning lights indicate when the system has failed.

CAUTION: Accumulators are under very high pressure. Never disassemble an accumulator without specific instructions to do so in a shop service manual. Incorrect disassembly procedures could cause the accumulator to fly apart and injure you.