Vacuum Booster Description

In order to properly service and repair available brake systems, a thorough understanding of the power assist systems is necessary. The vacuum assist diaphragm assembly multiplies the force exerted on the master cylinder piston in order to increase the hydraulic pressure delivered to the wheel cylinders or calipers which decreases the effort necessary to obtain acceptable stopping performance.
Vacuum assist units get their energy by using the pressure differential between engine vacuum and atmospheric pressure. A piston, cylinder and flexible diaphragm utilize this pressure differential to provide brake assistance. The booster cylinder consists of two chambers, one on each side of the diaphragm, both are maintained at engine vacuum, until the brake pedal is depressed. When the brake pedal is depressed atmospheric pressure is allowed to enter on one side of the diaphragm. The resulting pressure differential between the two sides of the diaphragm forces the diaphragm toward the engine vacuum side, this force is then used to assist in braking.
Brakes will operate even if the power unit fails. This means the conventional brake system and the power assist system are completely separate. Troubleshooting conventional and power assist systems are exactly the same until the power unit is reached. As with conventional hydraulic brakes, a spongy pedal still means air is trapped in the hydraulic system. Power brakes give higher line pressure, making leaks more critical.