TIAGO TIGOR BSVI - SERVICE MANUAL

BRAKES 11 7. VACUUM BRAKE BOOSTER General: Vehicle is equipped with a mechanical vacuum booster to assist driver’s pedal effort. This is achieved by using vacuum from engine inlet manifold (for petrol engines) or vacuum pump (for diesel engines). A diaphragm is provided between the two shells of the booster, the difference of pressure on two sides of diaphragm (one side vacuum and another side atmospheric pressure) gives mechanical advantage. This amplifies the driver’s pedal effort while braking. The unit is connected to the brake pedal and the master cylinder. The vacuum booster input push rod is connected to brake pedal while the output push rod operates the master cylinder. If failure of vacuum occurs, the two push rods act as a single rod and the brakes will work in the conventional manner, but more pedal effort will be required and the fatigue of the driver will increase. NOTE: The booster assembly and TMC assembly are coupled with the help of fasteners. The mating dimensions of booster and TMC are factory set hence: 1. Do not alter the height of the output rod of the vacuum booster unit at any stage and ensure booster output rod is correctly aligned to the primary piston bore during assembly of TMC to the booster. 2. Once the booster assembly is removed from vehicle, at the time of refitting on vehicle, the gasket at the mounting face of the booster should be replaced with a new one. 3. Booster assembly is a non-serviceable unit and it should never be tampered with. 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 for- ward, 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.