Villager V6-181 3.0L SOHC VIN 1 EFI (1997)
Vacuum Brake Booster: Description and Operation
Power Brake Booster (Part 1 Of 2)
Power Brake Booster (Part 2 Of 2)
The power brake booster is located on the bulkhead in the LH side of the engine compartment, between the brake pedal and the brake master cylinder. It
uses engine vacuum for operation. The applied force created when the brake pedal is depressed is mechanically increased with the assistance of the
engine vacuum. Power brakes reduce the amount of pedal pressure necessary to stop the vehicle, as well as reducing brake pedal travel (the distance
from the release position to full brake application). As a result, the brake pedal is approximately the same distance from the floor as the accelerator.
The power brake booster is divided into two chambers by a rubber diaphragm. The A-chamber is on the master cylinder side (front) of the power brake
booster, and the B-chamber is on the brake pedal side (rear) of the power brake booster. A vacuum exists in both chambers when the brake pedal is in the
released position. When the brake pedal is depressed, the air control valve is forced against the booster piston seat. This action closes the vacuum valve
between the two chambers and the vacuum to the B-chamber (rear) is cut off. As the brake pedal is further depressed, the air control valve allows
atmospheric pressure to enter the B-chamber. This creates a pressure difference between the two chambers. This pressure difference causes the
diaphragm, booster piston, and the air control valve to move forward into the A-chamber vacuum, pushing the piston rod into the brake master cylinder
and applying pressure to the braking system.
When the brake pedal is released, the booster piston return spring forces the diaphragm, booster piston, and air control valve rearward, closing the air
intake valve and opening the vacuum valve between the two chambers. A common vacuum now exists between the two chambers again.
A power brake booster check valve is located between the brake booster hose on the intake manifold side and the vacuum hose on the power brake
booster side. The power brake booster check valve closes when the engine is shut off. The closed power brake booster check valve traps engine vacuum
in the power brake booster. This reserve vacuum allows several assisted brake applications without the engine running.
If any of the vacuum-related parts fail, causing the power brake booster to fail, the brakes will still operate. The force of the brake pedal being depressed
will still be transferred through the control valve, booster piston, piston rod, and on to the brake master cylinder. The brake system will still operate, but
