The present invention relates to a pneumatic booster used in a brake system of a vehicle or the like.
A known pneumatic booster is provided with a constant-pressure chamber where a negative pressure constantly acts, and a variable-pressure chamber whose pressure can be controlled by a switching operation of a valve. When force is applied to an input shaft connected to a brake pedal, the valve is actuated, so that the atmospheric pressure acts on the variable-pressure chamber. Consequently, force is applied to a diaphragm that divides the constant-pressure chamber and the variable-pressure chamber, and the force is transmitted to an output shaft. When the force applied to the input shaft is removed, the valve allows the constant-pressure chamber and the variable-pressure chamber to communicate with each other. Thus, the force applied to the output shaft is canceled.
In the conventional pneumatic booster, the switching valve has a readily deformable portion and hence suffers from the disadvantage that when the pressure difference between the constant-pressure chamber and the variable-pressure chamber is large, air leakage occurs, and thus the desired operation cannot satisfactorily be performed. This problem is particularly remarkable in a pneumatic booster of the type in which pressurized air is supplied to the variable-pressure chamber.