Patent ID: 7591134

Claim:
A vacuum booster ( 1 ) of a brake device ( 50 ) including: a master cylinder ( 10 ) having a piston ( 13 a ) slidably fitted in a cylinder ( 12 a ) having a port ( 48 a ) communicating with a reservoir ( 46 ), so that the port ( 48 a ) is closed by forward movement of the piston ( 13 a ) to transmit liquid pressure from the cylinder ( 12 a ); the vacuum booster ( 1 ) to boost a force for operating a brake to push and move the piston ( 13 a ); a wheel cylinder ( 51 ) supplied with the liquid pressure from the master cylinder ( 10 ) to operate a braking unit ( 52 ) of each of the wheels ( 54 ); and an liquid pressure generating unit ( 53 ) to pump and supply brake liquid stored in the reservoir ( 46 ) to the wheel cylinder ( 51 ) through the port ( 48 a ) by using a pump ( 63 a ), wherein the vacuum booster comprising: a booster shell ( 2 ) defining a pressure chamber ( 3 ) therein; a partition member ( 4 ) provided in the booster shell ( 2 ) so as to be movable forwards and backwards, and partitioning the pressure chamber ( 3 ) into a variable pressure chamber ( 6 ) and a negative pressure chamber ( 5 ); a valve piston ( 8 ) coupled to the partition member ( 4 ); an input member ( 32 ) provided in the valve piston ( 8 ) so as to be movable forwards and backwards relative to the valve piston ( 8 ); a valve mechanism ( 30 ) including an atmospheric valve seat ( 21 a ) and a negative pressure valve seat ( 8 i ) respectively provided in the input member ( 32 ) and the valve piston ( 8 ), an atmospheric valve ( 31 b ) removably contacting the atmospheric valve seat ( 21 a ) to communicate or isolate the variable pressure chamber ( 6 ) with or from an atmosphere, and a negative pressure valve ( 31 a ) removably contacting the negative pressure valve seat ( 8 i ) to communicate or isolate the variable pressure chamber ( 6 ) with or from the negative pressure chamber ( 5 ); an output member ( 14 ) provided in the valve piston ( 8 ) so as to be movable forwards and backwards relative to the valve piston ( 8 ) and to push and move the piston ( 13 a ) with forward movement of the valve piston ( 8 ); reaction applying means ( 20 ) for transmitting part of reaction force that is transmitted from the piston ( 13 a ) to the output member ( 14 ) to the input member ( 32 ); and stop position shifting means ( 70 ) for shifting a stop position of the piston ( 13 a ), at which the piston ( 13 a ) is prevented from being moved backwards by the output member ( 14 ) when the valve piston ( 8 ) is at a non-operating position, such that the stop position of the piston ( 13 a ) at high temperature is set ahead of the stop position at low temperature, to reduce an area with which the port ( 48 a ) and the cylinder ( 12 a ) communicate with each other.