Abstract: A brake booster includes reaction transmitting means comprising a first and a second retainer, a coiled spring disposed between the both retainers, and a stop which prevents the both retainers from being spaced from each other beyond a given distance. The brake booster also includes a valve mechanism comprising an annular first valve seat formed on the inner periphery of the valve body, a second valve seat formed on the rear portion of the valve plunger at a location radially inward of the first valve seat, and a valve element adapted to be seated upon either valve seat. A backup plate having a diameter less than the internal diameter of the first valve seat is embedded into the valve element. This arrangement allows a sufficient hysteresis to be obtained in a region where the valve element causes an expansion and shrinkage of the coiled spring in the reaction transmitting means to provide a better brake feeling than a conventional arrangement.

Abstract: A valve mechanism for a brake booster includes a valve element and a valve seat, either of which includes a first seat through which an orifice passage is formed and a second seat. Under an intermediate load condition when a forward movement of an input shaft ceases, the first seats become engaged with each other and the orifice passage is effective to maintain a communication therethrough. Subsequently when the valve element and the valve seat are more strongly abutting together, the second seats engage to interrupt the communication.