Abstract: In a brake pressure control system provided with a discharge valve in its discharge line for the control fluid from a control chamber, the discharge valve having an on-off valve for opening and closing the discharge line so that when the discharge valve is opened, the control fluid pressure in the control chamber will be lowered to reduce directly or indirectly the braking force on the vehicle, the present invention provides a system having a second discharge valve arranged in series in the discharge line. With this system, even if one of the valves should fail to close the discharge line, the other valve will act to prevent the leakage of the control fluid pressure in the control chamber.

Abstract: A brake pressure control apparatus comprises a master cylinder (2), an auxiliary hydraulic power source (10), a booster (3), a comparator member (30), an inlet valve (28), a pressure raising member (31) and a limiter member (32). The comparator member (30) determines whether a fluid pressure in a fluid path connecting a boost chamber of the booster (3) with wheel brakes (8 and 9) is lower than a predetermined value. The inlet valve (28) cuts off flow of the fluid toward the wheel brakes (8 and 9) when it is determined that the fluid pressure in the fluid path connecting the boost chamber with the wheel brakes (8 and 9) is lower than the predetermined value. The pressure raising member (31) raises a brake pressure to be applied to the wheel brakes (8 and 9), based on a fluid pressure from the master cylinder. The limiter member (32) limits operation of the pressure raising member (31).