PURPOSE: To carry out control of brake hydraulic pressure in a high degree of safety, by urging valve members with the use of both air spring which determines the intitial pressures of pressure reducing operation of pressure reducing valves in accordance with a loaded weight of a vehicle, and mechanical spring.

CONSTITUTION: Pressure reducing valves 51, 52 for proportionally reducing outlet hydraulic pressures at the outlet ports P1, P2 of a master cylinder M and for transmitting the thus reduced hydraulic pressures to rear wheel brakes Br1, Br2, are disposed in oil passages L1, L2 communicating between the outlet ports P1, P2 and the rear wheel brakes Br1, Br2, and the initial pressures of pressure reducing operation of the pressure reducing valves P1, P2 are determined in accordance with a loaded weight of a vehicle. An air spring in a suspension system and a mechanical spring 16 which also determines the initial pressures of pressure reducing operation of the pressure reducing valves upon a failure of the air spring system, are coupled to valve members 9 so that the valve members 9 are urged in the valve opening direction. Thereby, an ideal control of brake hydraulic pressure may be obtained, and even if the air spring system fails, hydraulic pressure to the rear wheel brakes may be reduced by determining the initial pressures of pressure reducing operation by means of the mechanical spring so that the locking phenomenon of the rear wheels may surely be prevented.