Vacuum Brake Booster: Description and Operation

The power brake unit is a vacuum assist diaphragm assembly which multiplies the force exerted on the master cylinder piston in order to increase the hydraulic pressure delivered to the wheel calipers. This effectively decreases the effort necessary to attain sufficient stopping performance.
Vacuum assist units are powered by the opposition of engine vacuum to atmospheric pressure. A piston, cylinder and flexible diaphragm utilize this energy to provide brake assistance. The diaphragm is balanced with engine vacuum until the brake pedal is depressed, allowing atmospheric pressure to unbalance the unit and apply force to the brake system.
The basic brake system will operate even if the power unit fails, although more effort is required at the pedal to achieve the same amount of braking performance attainable with a functional power assist unit.