Conventional vacuum-operated brake power boosters as a rule have a housing in which there is an axially movable wall which is adapted to be advanced and retracted within the vacuum housing to divide the inner space of the housing into two chambers, i.e., one chamber on each side of the axially movable wall. By means of the two chambers, a pressure difference can be generated on the opposite sides of the movable wall, which pressure difference results in the movable wall being shifted in the direction of the lesser pressure. A vacuum-operated brake power booster of this type furthermore comprises a control valve which is actuated through the brake pedal and which controls the amount of vacuum acting on the one side of the axially movable wall. As a rule, the axially movable wall is connected to the brake master cylinder of a hydraulic braking system of the automotive vehicle in such a manner that when the pressure difference has its effect on the movable wall forcing a movement of the movable wall, brake fluid is displaced from the brake master cylinder into the hydraulic braking system to apply the brakes of the vehicle.
In the design of braking systems for automotive vehicles, it is expedient to configure the vacuum-operated brake power booster in a manner so that the brake response phase in which the driver moves the brake pedal and interrupts the connection of flow between the two sides of the axially movable wall by means of the control valve is reduced to a minimum.
A brake power booster of the type generally disclosed herein is known from German printed and published patent application 3,042,096 Al. In this a wedge is provided which is seated with play both in the control valve piston and in the control valve body. The wedge is adapted to abut against the housing of the brake power booster in such a way that in the release position of the brake, the vacuum valve seat is lifted a minute distance from the poppet valve. This known brake power booster has the disadvantage that the potential lost motion cannot be completely reduced to zero.