Power Brake Assist: Description and Operation

In order to properly service power brake systems, a thorough understanding of the various power assist systems is necessary. These systems provide a mechanical assist, acting to increase the force applied to the master cylinder piston by the driver while decreasing the effort necessary to obtain acceptable stopping performance. Two basic types of power assist mechanisms are used: vacuum assist diaphragm assemblies which use engine vacuum and in some cases vacuum from an external vacuum pump, and hydraulic assist assemblies which use hydraulic pressure developed by an external pump (usually the power steering pump).
Vacuum assist units are similar in operation, using vacuum applied to a diaphragm or piston in opposition to atmospheric pressure in order to increase the force applied to the master cylinder piston. However, two types of vacuum assist assemblies are used, the air suspended type and the vacuum suspended type. Air suspended units are under atmospheric pressure until the brakes are applied, then engine vacuum is admitted causing the piston or diaphragm to move and apply force to the master cylinder pushrod. Vacuum suspended types are balanced with engine vacuum until the brake pedal is depressed, allowing atmospheric pressure to unbalance the unit and apply force to the brake system.
Hydraulic assist units incorporate a basic open center spool valve, hydraulic cylinder and an accumulator assembly incorporated into a common housing. When the brake pedal is depressed the hydraulic piston moves forward closing off fluid relief passages, the spool valve applies fluid pressure to the rear of the piston, and pressure applied to the piston is transmitted to the brake master cylinder through a pushrod. When the brake pedal is released, fluid pressure behind the piston is exhausted through the open center of the spool valve to a return line and the pump reservoir, and the spool valve diverts fluid pressure away from the hydraulic piston. The accumulator stores sufficient fluid under pressure to provide two or three power assisted stops in the event that pressure from the pump is interrupted. The design of the hydraulic cylinder allows the brakes to be applied without power assist, however, greater pedal effort is required to apply the brakes manually.