Negative pressure type booster

A negative pressure type booster comprising:

• a housing;
• a movable wall mounted in the housing to be movable frontward and rearward, the movable wall dividing the interior of the housing into a constant-pressure chamber and a variable-pressure chamber;
• a power piston connected to the movable wall and being movable frontward and rearward;
• an input member provided in the power piston to be movable frontward and rearward relative to the power piston, the input member receiving an external operation force applied thereto in a frontward direction;
• an output member for outputting, to an external device, a frontward thrust force generated by the power piston;
• a control valve mounted in the power piston and comprising an atmospheric control valve portion and a vacuum control valve portion, the atmospheric control valve portion facing frontward, toward an atmospheric valve seat provided on the input member, and adapted to establish and shut off communication between the variable-pressure chamber and the atmosphere in cooperation with the atmospheric valve seat, and the vacuum control valve portion facing frontward, toward a vacuum valve seat provided on the power piston, and adapted to establish and shut off communication between the variable-pressure chamber and the constant-pressure chamber in cooperation with the vacuum valve seat;
• first biasing means for biasing the control valve frontward toward the atmospheric valve seat and the vacuum valve seat;
• second biasing means for biasing the input member rearward;
• and
• a stopper for limiting a quantity of movement of the input member relative to the power piston effected by biasing force of the second biasing means;
• wherein the atmospheric valve seat is formed on an annular portion of the input member extending radially outward;
• the annular portion and an annular diaphragm located frontward of the annular portion and mounted to the input member define an annular atmospheric chamber;
• the atmospheric chamber communicates with the atmosphere via an air hole formed in the annular portion or in the input member;
• and the diaphragm is supported from the front by the power piston.