SAFARI BSVI - SERVICE MANUAL

BRAKES (Non EPB) 12 A. WORKING PRINCIPLE OF VACUUM BOOSTER: 1. Brakes OFF The diaphragm is fully retracted and is held against the rear shell by the diaphragm return spring. The in- put rod assembly is also fully retracted by the brake pedal return spring. With the input rod in this position, the vacuum port is open and the vacuum is applied on both the sides of the diaphragm 2. Brakes Applied When the brake pedal is depressed, the input rod assembly moves forward inside the diaphragm plate until the control valve closes the vacuum port; at this stage vacuum is still present on both the sides of the diaphragm. As the input rod continues to move forward, the control plunger moves away from the control valve, opening the atmospheric port, which is formed between these two parts. Air enters the rear shell behind the diaphragm and the effect of atmospheric pressure assisted by the valve rod helps in pushing the diaphragm plate forward and thus the out- put rod actuates the TMC plunger. 3. Brakes Held ON When the brake pedal is held on, the diaphragm will momentarily continue to move forward and thus compresses the outer edges of the reaction disc. This movement causes the center of the disc to extrude, pressing back the input rod and thus closing the atmospheric port. Thus the output force is balanced against that of the input force. Further movement of the brake pedal either opens the vacuum port or the atmospheric port depending on whether the brake pedal is released or depressed. 4. Brakes Released Immediately after the brake pedal is released, the vacuum port is opened and the atmospheric pressure in the rear chamber is extracted into the front chamber and from there to the inlet manifold via the non- return valve. The atmospheric port remains closed whilst the input rod assembly returns to its original position as shown in figure. The diaphragm is then again ‘suspended’ in vacuum until the brake pedal is depressed.