Rest mode
The common vacuum operated power brake booster is made up of a vacuum valve, atmospheric valve, a rubber covered steel diaphragm and a stamped steel housing.
The rest mode is the mode the booster is operating in any time the engine is running and the brakes are not applied. During this time the vacuum is equal on either side of the steel diaphragm inside the booster housing, centering the diaphragm in the housing. This is also the normal mode of operation while driving, when no brake input is given by the driver.
Applied mode
When the brake pedal is applied, the vacuum is shut off from the rear chamber, and the chamber is vented to allow atmospheric pressure to enter the rear chamber. This happens when the vacuum valve in the booster is closed and the air valve opens. The difference in pressure between the two halves causes the diaphragm to deflect. Increasing the amount of pressure the driver applies to the pedal increases the volume of atmospheric pressure allowed into the diaphragm, causing additional pressure to be applied to the master cylinder.
Hold mode
During the hold mode, when the driver's brake input stabilizes, the diaphragm continues to deflect until both valves (atmospheric and vacuum) close. This prevents further deflection of the diaphragm, and holds steady pressure on the brake system .
Release mode
When the brake pedal is released by the driver, the atmospheric valve closes and the vacuum valve opens allowing the pressure to equalize on both sides of the diaphragm. A combination of spring pressure and hydraulic pressure returns the diaphragm to center. When the diaphragm centers and vacuum equalizes, we return to rest mode.
