Tracker 4x4 L4-1.6L VIN 6 (1995)
Vacuum Brake Booster: Description and Operation
Fig. 1 Exploded View Of Tracker Brake Booster
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.
This unit, Fig. 1, uses a single vacuum diaphragm with an effective diameter of 7 inches to mechanically increase brake application force with
engine vacuum.
The diaphragm and plate assemblies use the pressure differential created by engine vacuum and atmospheric pressure to assist in applying the
