Patent ID: 6755116
Filing Date: 2004-06-29
Classification: B60T

Abstract:
A vacuum type brake booster comprising:a housing for defining at least one pressure chamber therein; a movable wall member disposed in the housing to divide the pressure chamber into a constant pressure chamber and a variable pressure chamber and to be movable in forward and backward directions with respect to the housing; a power piston connected to the movable wall member; an input member disposed in the power piston to be movable in forward and backward directions with respect to the power piston; an output member for outputting a propulsion force of the power piston in response to the movement of the movable wall member outside the booster; an annular valve seat for atmosphere air outflow control disposed in the power piston to be directed rearwardly; a valve mechanism having a movable portion opposed to the annular valve seat for atmospheric air outflow control and a fixed portion secured air-tightly to the power piston; the annular valve seat for atmospheric air outflow control being secured to a valve seat member which is movable in forward and backward directions with respect to the power piston; further, a holding means for holding the valve seat member at a predetermined front position in the case that an advanced distance of the input member with respect to the power piston is less than a predetermined value; a movable means for shifting the valve seat member up to a predetermined distance at a rear position in the case that an advanced distance of the input member with respect to the power piston is more than the predetermined value; and a returning means for returning the valve seat member at the predetermined front position in the case that the power piston is in a predetermined position with respect to the housing; wherein the holding means is disposed at the front end inner peripheral side of the power piston and has an engagement portion engageable with the valve seat member.