If were dealing with an Otto-engine controlled by a throttle-flap, then the vacuum in the inlet pipe is mostly sufficient. All other engines - including the Diesel - require a vacuum-pump. In both cases, when the engine is
running, the vacuum is directed via a check-valve into both chambers of the power brake. These result, when the entire metal pot is divided by a movable membrane. Depending on how hard the brake pedal
is pressed, more or less atmospheric pressure is let into the right hand chamber through an air filter (see arrow!), thus supporting the braking action of the driver.
|The suction-vacuum/vacuum pump amplifies the pedal pressure.
Thereby, the diameter (measured in inches) determines
how much brake support can be given. If in heavier cars, there is not enough space for a larger diameter, then three or even four chambers may be provided. If a hydraulic system already exists, then the support can
be taken care of by the much smaller hydraulic-power-brake.
|Release position: Vacuum in both the left and the right chamber.
The brake pedal is connected by the piston rod to the brown plunger and the black sealing-ring. Both the plunger and the sealing-ring are a small distance from their contact surfaces. This causes a vacuum in both
chambers. The connection to the outside is interrupted because the brown plunger has made contact with the sealing ring.
|Partial-braking: Right hand chamber is partially ventilated.
In the case of a partial-braking, the piston-rod is shifted to the left. The brown plunger is lifted from the sealing-ring. The green spring moves it and consequently, separates the two chambers from each other. At the
same time, the right chamber is (partially) filled with atmospheric pressure. This only lasts until the membrane, with the complete control-housing, has moved so far to the left that the brown plunger once again
contact with the sealing-ring. Thus, the pressure increase in the right chamber depends on the brake pedal force.
|Full-braking: Right hand chamber is filled with atmospheric pressure.
In the case of full-braking, the black sealing-ring cannot be moved further to the left. Thereby, it is permanently lifted from the brown plunger. In the right chamber the full atmospheric pressure is effective.