Patent ID: 7685926

Claim:
A negative pressure booster comprising; a valve body which is reciprocally disposed inside a shell, a power piston which is mounted in the valve body and partitions an interior of the shell into a constant pressure chamber into which negative pressure is introduced and a variable pressure chamber into which atmosphere is introduced at a time of operating the negative pressure booster, a valve plunger which is connected to an input shaft and is slidably disposed inside the valve body, a vacuum valve which controls communication or interruption between the constant pressure chamber and the variable pressure chamber, an atmospheric valve which controls interruption or communication between the variable pressure chamber and at least atmosphere due to operation of the valve plunger, and a stroke shortening mechanism which shortens a manipulation stroke quantity of the input shaft in an output region with an output larger than a given output, than the manipulation stroke quantity of the input shaft in the output region with the output larger than the given output when the manipulation stroke quantity of the input shaft is changed at a change rate of the manipulation stroke quantity of the input shaft with respect to the output in an output region with an output equal to or below the given output, wherein the stroke shortening mechanism is atmospheric valve opening quantity increasing means which is operated in the output region with the output larger than the given output and increases a valve opening quantity of the atmospheric valve larger than the valve opening quantity during a usual operation, and the operation of the atmospheric valve opening quantity increasing means is controlled in response to pressure of the variable pressure chamber corresponding to the input.