NEGATIVE PRESSURE TYPE BRAKE BOOSTER

A negative pressure type booster, comprising a valve seat member (40), a spring (47) energizing the valve seat member (40), and a holding member (52) having an engaging part (52b) disposed on a power piston (22) movably only in radial direction and engaged with the valve seat member (40) and a tapered part (52a) in contact with a plunger (37), whereby, when the amount of movement of an input member (27) relative to the power piston (22) is larger than a specified value, the input member (27) is brought into contact with the tapered part (52a) and moves the holding member (52) in radial direction to disengage the holding member (52) from the valve seat member (40) in order to allow the valve seat member (40) to be moved rearwards.