Vehicle braking system and master cylinder

A master cylinder used for a vehicle braking system comprising a booster for assisting an input from a brake pedal and outputting the input with an assist force, the booster having a full-load point at which the assist force is no longer provided, the master cylinder comprising a stepped piston moved by the output of the booster, the master cylinder comprising:

• the stepped piston moved by the booster;
• a stepped cylinder comprising a small-diameter pressure chamber for supplying a hydraulic pressure to wheel cylinders and a large-diameter pressurized chamber for supplying the hydraulic pressure to the small-diameter pressure chamber, the small-diameter pressure chamber and the large-diameter pressurized chamber being formed by insertion of the stepped piston;
• and
• a pressure-reducing valve to be opened at a predetermined valve-opening pressure by the hydraulic pressure in the small-diameter pressure chamber and the 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,
• wherein the valve-opening pressure for the pressure-reducing valve is set so that the pressure-reducing valve is opened in case of a failure of the booster when the hydraulic pressure in the small-diameter pressure chamber is higher than a hydraulic pressure obtained with a 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.

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 through actuation of a booster caused by a brake pedal, 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 hydraulic pressure being supplied from the small-diameter pressure chamber to wheel cylinders;
• and
• a pressure-reducing valve being opened at a predetermined valve-opening pressure by a hydraulic pressure in one of the small-diameter pressure chamber and the large-diameter pressurized chamber to bring the large-diameter pressurized chamber into communication with a reservoir so as to gradually reduce the hydraulic pressure in the large-diameter pressurized chamber along with an increase in the hydraulic pressure in the small-diameter pressure chamber,
• wherein the pressure-reducing valve is set so as to be opened when the hydraulic pressure in the small-diameter pressure chamber is higher than 2 MPa and lower than 10 MPa to make the hydraulic pressure in the large-diameter pressurized chamber equal to an atmospheric pressure along with the increase in the hydraulic pressure in the small-diameter pressure chamber after the pressure-reducing valve is opened.