Impreza RS Coupe AWD F4-2.5L SOHC (1999)
Vacuum Brake Booster: Description and Operation
Operation
Brake Booster "Off"
The plunger valve comes in contact with the poppet valve so that atmospheric air passing through the filter and silencer is shut out by the atmospheric
valve (of the poppet valve).
The plunger valve is moved to the key at the right by the return spring so that the poppet valve is held at the right. Since the vacuum valve of the valve
body and the poppet valve are kept away from each other, passage A is linked with passage B and the constant-pressure chamber is also linked with the
variable-pressure chamber. At this point, pressure differential does not occur between the two chambers; the diaphragm plate is moved back to the right
by return spring tension.
Brake Booster "on"
When the brake pedal is depressed, the operating rod pushes the plunger valve so that the poppet valve comes in contact with the vacuum valve of the
valve body. This shuts off the circuit between passages A and B. as well as the circuit between the constant- and variable-pressure chambers.
Further movement of the plunger valve moves the atmospheric valve away from it so that atmospheric air is directed to the variable-pressure chamber via
passage B. This produces a pressure differential between the constant- and variable-pressure chambers.
As a result, the diaphragm and its plate are moved to the left as a single unit.
