Brake pressure generator having means for controlling pressure independent of operation of brake operating member

A hydraulically operated anti-lock brake system for a motor vehicle having a wheel comprising:

• a master cylinder having a pressure piston which partially defines a pressure chamber and which is advanced to generate a fluid pressure in said pressure chamber in response to an operation of an operating member;
• a wheel cylinder operated by said fluid pressure to apply a brake to said wheel;
• slip detecting means for generating an output signal indicative of the presence of an excessive amount of slipping of said wheel on a road surface;
• an inhibiting device defining a non-operated state and an operated state, said inhibiting device allowing said pressure piston to be advanced in response to the operation of said operating member when in a non-operated state, and inhibiting said pressure piston from being advanced in response to the operation of said operating member when in an operated state;
• a pressure changing device having a pressure changing chamber communicating with said pressure chamber, and operable to change a volume of said pressure changing chamber and thereby change the pressure in said pressure chamber;
• and
• a controller for normally placing said inhibiting device in said non-operated state when the output signal does not indicate the presence of an excessive amount of slipping of said wheel, and for placing said inhibiting device in said operated state and controlling said pressure changing device when the output signal of said slip detecting means indicates the presence of an excessive amount of slipping so as to change the pressure in said pressure chamber for maintaining an amount of slipping of said wheel within an optimum range,
• said controller holding said inhibiting device in said operated state to inhibit an advancing movement of said pressure piston in response to the operation of said operating member until completion of an operation for controlling the pressure in said pressure chamber so as to maintain said amount of slipping of said wheel within said optimum range,
• said pressure changing device comprising (a) a screw device including an externally threaded member and an internally threaded member which engage each other, one of said externally and internally threaded members being a rotatably immovable threaded member prevented from rotating about an axis thereof, (b) a drive device for rotating the other of said externally and internally threaded members, the other of said externally and internally threaded members being a rotatably movable threaded member, and (c) a pressure changing piston axially movable with one of said externally and internally threaded members, for changing the volume of said pressure changing chamber.