Vacuum Booster Operation
Vacuum brake assist units use a large round flexible diaphragm enclosed in an assembly to create two distinct vacuum chambers. The peddle side of the diaphragm has two valves attached to the peddle arm. These valves work in unison to control booster operation.
A brake booster needs the right amount of vacuum to operate properly. This is typically engine manifold (17-21Hg) vacuum and on some engines, particularly diesels an auxiliary pump man be used. This vacuum is applied to both sides of the booster. The peddle side and the master cylinder side. 
As the driver presses the brake peddle the vacuum valve blocks the vacuum source located on the peddle side of the booster and at the same time the air valve moves allowing atmospheric pressure in. Sometimes a slight hissing noise can be heard as the air rushes past the filter, through the air valve and into the chamber.
This collapse in vacuum on the peddle side accompanied with the vacuum source still applied to the master cylinder side produces a powerful brake assist. This is because of the flexible diaphragm located in the center of the assembly. A rod or "power piston" is connected to this diaphragm. This pushrod is mechanically connected to the diaphragm and through the entire unit. One end is connected directly to the master cylinder and the other is connected to the brake peddle. This is why vacuum brake booster failure will not result in a complete loss of braking action. There is always a mechanical connection between the two.
When the driver releases the brake peddle a return spring located in the center of the master cylinder side of the booster assembly returns the diaphragm to its resting position. While the spring moves the diaphragm the air and vacuum valves are also returned to their normal positions returning vacuum to both sides of the diaphragm. This is known as it's balanced or at rest position. Brake booster failure will leave a vehicle with a hard brake peddle making it much more difficult to stop.
