Master cylinder

A master cylinder comprising:

• a cylinder;
• a piston slidably provided in the cylinder, the piston and the cylinder defining a pressure chamber for generating a fluid pressure in front of the piston, the cylinder and an outer circumferential surface of the piston defining a fluid supply chamber which communicates with a reservoir;
• an annular cup seal held on an inner circumferential surface of the cylinder, the cup seal being adapted to slide against the outer circumferential surface of the piston to separate the fluid supply chamber from the pressure chamber;
• and
• a relief port formed in the piston, the relief port having opposite openings, one of the openings being open in the outer circumferential surface of the piston and the other opening being always open to the pressure chamber, so as to be capable of providing communication between the pressure chamber and the fluid supply chamber,
• wherein the piston has a smaller-diameter portion and a forwardly facing control taper surface positioned rearwardly of the opening of the relief port in the outer circumferential surface of the piston and connecting to the smaller-diameter portion, and
• the cup seal is positioned so that an inner peripheral portion thereof resides over the smaller-diameter portion when the piston is in a rest position, and the cup seal is sized so that the diameter of the inner circumferential surface of the cup seal at the rear end thereof is smaller than the diameter of the control taper at the rear end of the taper.