housing 21 in which a power piston 31 an operating piston 32 a control valve 40 and control pistons 46 and 47 are assembled the housing 21 is provided thereon with first and second inlet ports 21a and 21b the first inlet port 21a is in communication with the master cylinder 10 and a port 51a of a cut off valve unit 50 through the pipe line p 1 the second inlet port 21b is in communication with a port 51b of the cut off valve unit 50 through a pipe line p 2 the housing 21 is also provided thereon with an outlet port 21c in communication with the wheel brake cylinders 12 through the pipe line p 3 an air inlet port 21d of the housing 21 is communicated through a pipe line p 4 with an air tank 13 which is connected to a source of compressed air not shown an air outlet port 21e of the housing 21 is communicated directly with the atmospheric air the housing 21 has a first stepped bore 22 a main bore 23 coaxial with the first stepped bore 22 and a second stepped bore 26 in parallel with the first stepped bore 22 the first stepped bore 22 is isolated from the main bore 23 and the second stepped bore 26 is communicated with the main bore 23 through a passage 23a the power piston 31 is slidably disposed through an annular sea member within the main bore 23 to form a power chamber r 2 in communication with a control chamber r 1 in the second stepped bore 26 by way of a passage 27 the power piston 31 urged toward the lower chamber r 2.