Hydraulic booster

A hydraulic booster having a boost chamber and means for receiving auxiliary dynamic pressure from an auxiliary power source in response to an operation of a brake pedal for generating a boost pressure proportional to a pedal force in said boost chamber for assisting a thrust of a master cylinder by said boost pressure, said hydraulic booster comprising a boost piston in said boost chamber, said boost piston having a rear end portion facing said boost chamber for receiving the thrust of said boost pressure to move in a forward direction for urging a master piston of said master cylinder also in a forward brake force applying direction;

• an input member movable in said boost chamber with respect to said boost piston for receiving input effective in a forward direction, said input being proportional to said pedal force, said input member receiving a rearwardly effective reaction force proportional to said boost pressure;
• pressure regulating means having an input directional control valve for controlling communication between said boost chamber and said auxiliary power source and an output directional control valve for controlling communication between said boost chamber and a reserve tank being open to the atmosphere for increasing said boost pressure by opening said input directional control valve and closing said output directional control valve when said input member is relatively positioned ahead of a prescribed position with respect to said boost piston while reducing said boost pressure by closing said input directional control valve and opening said output directional control valve when said input member is relatively positioned behind a prescribed position with respect to said boost piston;
• a housing containing said boost chamber and said boost piston, said input member and said pressure regulating means in its interior;
• input member positioning means for bringing a rear end of said input member into contact with said housing or a stop member anchored to said housing for stopping a rearward movement of said input member when said pedal force is zero;
• and boost piston positioning means for balancing a force for rearwardly pushing said boost piston with a force for frontwardly pushing said boost piston by said boost pressure for stopping a rearward movement of said boost piston when said pedal force is zero, so that travel loss and a respective waste of auxiliary dynamic pressure are reduced to a minimum and a spongy brake pedal feeling is avoided.