Patent ID: 7021724

Claim:
A vacuum booster comprising: a booster shell; a booster piston accommodated in the booster shell to divide an interior of the booster shell into a front vacuum chamber connected to a vacuum source and a rear operating chamber; a valve cylinder slidably supported at a rear wall of the booster shell to be connected to the booster piston; an input rod movable forward and backward, an input return spring that biases the input rod in a retreat direction, and a control valve that changes over communication of the operating chamber with one of the vacuum chamber and atmosphere in accordance with forward and backward movement of the input rod, which are placed in the valve cylinder, the control valve being constituted by an atmosphere introduction valve seat that is formed on a valve piston which moves in a manner operatively connected with the forward and backward movement of the input rod, a vacuum introduction valve seat that is connected to the valve cylinder to move in a manner operatively associated with forward and backward movement of the valve cylinder, and a valve element that is biased by a spring in a direction in which it is seated on the atmosphere introduction valve seat and the vacuum introduction valve seat, cuts off communication of the operating chamber with the atmosphere and the vacuum chamber when the valve element sits on both the valve seats, allows the operating chamber to communicate with the atmosphere when the atmosphere introduction valve seat opens following the forward movement of the input rod, and allows the operating chamber to communicate with the vacuum chamber when the vacuum introduction valve seat opens following the retreat of the input rod; and a reaction force mechanism that is interposed between the valve cylinder with the input rod and an output rod that is slidably supported at the booster shell, and that transmits a combined force of a manipulation input to the input rod and a propulsive force of the booster piston by atmospheric pressure difference between the operating chamber and the vacuum chamber to the output rod, wherein a valve piston is fitted onto an input piston of the reaction force mechanism which is connected to the input rod, so that the valve piston is capable of sliding forward from a predetermined retreat position with respect to the input piston, wherein a set spring that biases the valve piston to the retreat position is provided between the input piston and the valve piston, and wherein locking means is provided between the valve piston and the valve cylinder to hold the atmosphere introduction valve seat in an open state by locking the valve piston, which moves forward with the input piston, at the valve cylinder at the time of emergency braking when the input piston moves forward by a predetermined stroke or more ahead of the valve cylinder.