Patent ID: 8807668

Claim:
A vehicle braking system, comprising: a master cylinder comprising: a small-diameter pressure chamber and a large-diameter pressurized chamber which are formed by a stepped piston inserted into a stepped cylinder, the stepped piston being moved to supply a hydraulic pressure from the large-diameter pressurized chamber to the small-diameter pressure chamber and to generate a hydraulic pressure in the small-diameter pressure chamber, the generated hydraulic pressure being supplied to wheel cylinders; and a pressure-reducing valve being opened at a predetermined valve-opening pressure by the hydraulic pressure in the small-diameter pressure chamber and a hydraulic pressure in the large-diameter pressurized chamber to bring the large-diameter pressurized chamber into communication with a reservoir so as to reduce the hydraulic pressure in the large-diameter pressurized chamber; a booster for assisting an input from a brake pedal to move the stepped piston with an assist force, the booster having a full-load point at which the assist force is no longer provided; detection means for detecting a failure of the booster; and pressure-intensifying means for generating hydraulic pressure by a hydraulic pressure source different from that of the master cylinder and, in case of the failure of the booster and a pressing force applied to the brake pedal being 500 N, supplying a predetermined hydraulic pressure higher than a hydraulic pressure generated in the small-diameter pressure chamber to the wheel cylinders, in order to compensate an insufficient amount of hydraulic pressure generated by the master cylinder, when the failure of the booster is detected by the detection means, wherein the valve-opening pressure for the pressure-reducing valve is set so that the pressure-reducing valve is opened when the hydraulic pressure in the small-diameter pressure chamber is higher than the predetermined hydraulic pressure supplied by the pressure-intensifying means in the case of the failure of the booster and the pressing force applied to the brake pedal being 500 N, and is lower than a hydraulic pressure obtained at a time when the booster reaches the full-load point, and wherein the master cylinder is configured to supply a hydraulic pressure generated in the large-diameter pressurized chamber until the hydraulic pressure in the master cylinder reaches the predetermined hydraulic pressure in case of the failure of the booster.