Negative pressure type booster

A negative pressure type booster comprising:

• a housing having a pressure chamber formed therein;
• a movable diaphragm mounted in the housing in a frontward/rearward movable condition and dividing the pressure chamber into a constant-pressure chamber and a variable-pressure chamber;
• a power piston connected to the movable diaphragm;
• an input member provided in the power piston to be moved frontward and rearward relative to the power piston and receiving an external operation force;
• an output member for outputting, to an external device, a thrust force generated by the power piston;
• a slide valve disposed between the power piston and the input member, and mounted coaxially to the power piston to be moved frontward and rearward;
• holding means for holding the slide valve at a predetermined frontward position when a frontward-movement quantity of the input member relative to the power piston is equal to or less than a predetermined value;
• moving means for moving the slide valve rearward by a predetermined quantity when the frontward-movement quantity of the input member relative to the power piston is greater than the predetermined value;
• return means for returning the slide valve to the predetermined frontward position when the power piston and the slide valve return to respectively predetermined positions relative to the housing;
• and
• a control valve mounted in the power piston and comprising an atmospheric control valve portion for establishing and shutting off communication between the variable-pressure chamber and the atmosphere in cooperation with an atmospheric valve seat provided on the input member, and a vacuum control valve portion for establishing and shutting off communication between the variable-pressure chamber and the constant-pressure chamber in cooperation with a vacuum valve seat provided on at least one of the power piston and the slide valve;
• wherein the holding means comprises a latch member that is mounted to the power piston in such a manner as to be linearly movable in a radial direction, and a biasing member for biasing the latch member radially inward;
• the latch member has a hook that can be engaged with and disengaged from a hook provided on the slide valve;
• and the hook of the latch member and the biasing member constitute the holding means.