During application, the reaction force against the valve plunger works against the driver to close the atmospheric port. With both the atmospheric and the vacuum ports closed, the power unit is in a holding position. It stays this way until increased pedal force re-opens the atmospheric port, or a drop in pedal force re-opens the vacuum port.
With the force on the pedal held constant, the valve returns to the holding position. But if the pedal is fully applied, the plunger moves away from the control valve to open the atmospheric port and give full power application.
When the brakes are released, vacuum returns to both sides of the diaphragm, so the spring releases the brakes.
When the engine is switched off or stops for any reason, no vacuum is available. The vacuum remaining in the booster, held by the non-return valve, will provide for at least one power-boosted application.
After this, the brakes will still operate, but without power assistance, they require more effort from the driver.
