Patent ID: 7263830

Claim:
A master cylinder comprising: a cylinder body in a cylindrical form having one end closed and including a discharge passage and a supply passage for a brake fluid, the supply passage being communicated with a reservoir; a piston slidably disposed in a slidable inner-diameter portion of the cylinder body, the piston and the cylinder body forming a pressure chamber therebetween for supplying a fluid pressure to the discharge passage; and a piston seal provided in a circumferential groove formed in the cylinder body, the circumferential groove being located adjacent to the slidable inner-diameter portion, the piston seal having an inner circumferential surface thereof in slidable contact with an outer circumferential surface of the piston and being capable of sealingly disconnecting the supply passage and the pressure chamber which is pressurized by means of the piston, wherein a large-diameter bore portion is formed so as to axially extend in a portion defining the pressure chamber in the cylinder body on a side opposite to the circumferential groove relative to the slidable inner-diameter portion, the large-diameter bore portion having an axis that is the same as an axis of the slidable inner-diameter portion and having a larger diameter than the slidable inner-diameter portion, and wherein a communication groove capable of allowing communication between the circumferential groove and the large-diameter bore portion is formed in an upper portion of the slidable inner-diameter portion, a point of maximum depth of the communication groove being located radially outward of a bottom of the circumferential groove.