PURPOSE: To simultaneously effect the rises of braking operations in two systems, by providing a bypass valve such that a bypass passage connecting the two systems is opened when the pressures of compressed air sources in the two systems are made not lower than a predetermined value.

CONSTITUTION: The first chamber 6 and the second chamber 9 of the compressed air source are connected respectively to inlet ports 27a, 27b of a double check valve 27. An outlet port 35 of the valve 27 is connected to a valve gear 37 through a pipe 36. A piston 46 for the valve gear 37 occupies a raised position when receiving a pressure fed through the valve 27. At the raised position of the piston 46, a small diameter passage 45 is separated from an O-ring 44, so that pressure chambers 40, 41 of two boosters 18, 19 are communicated to each other through a crooked passage 42. Therefore, rises of braking actions in the two systems are effected simultaneously. When the pressure in either one of the first and second chambers 6, 9 is lowered, the piston 46 is moved downward to come into contact with the O-ring 44. Accordingly, since the pressure chambers 40, 41 are not communicated with each other, a braking action on the normal side is secured.