Patent ID: 6973784

Claim:
A hydraulic master cylinder, comprising: a cylinder main body internally formed with a piston mount space extending in a longitudinal direction and formed at one end thereof along the longitudinal direction with a supplying/discharging opening for supplying and discharging a hydraulic liquid into and from the piston mount space; a piston mounted in the piston mount space in such a manner as to be movable by a restricted stroke along the longitudinal direction; and a seal ring mounted on an outer circumferential portion of the piston in such a manner as to project more radially outward than the outer circumferential surface of the piston and held in sealing contact with the inner circumferential surface of the cylinder main body defining the piston mount space, thereby cutting off the flow of the hydraulic fluid at a sealing-contact position, wherein: the cylinder main body includes an auxiliary supplying/discharging passage communicating sideways with the piston mount space; the inner circumferential surface of the cylinder main body and the outer circumferential surface of the piston are so shaped as to define a communication path for communicating a pressure chamber, which is a part of the piston mount space closer to the supplying/discharging opening than the seal ring, and the auxiliary supplying/discharging passage with the piston moved to a rearmost position most distanced from the supplying/discharging opening at the outer side of the seal ring; the inner circumferential surface of the cylinder main body defining the piston mount space includes a pressure-chamber side inner circumferential surface having such an inner diameter that the outer circumferential surface of the seal ring is in sealing contact therewith over the entire circumference and a communicating inner circumferential surface adjacent to a side of the pressure-chamber side inner circumferential surface opposite from the supplying/discharging opening and shaped such that at least a part thereof along circumferential direction is radially separated form the outer circumferential surface of the seal ring; an inner space of the communicating inner circumferential surface communicates with the communication path; the position of the communicating inner circumferential surface is set such that the outer circumferential surface of the sealing ring faces the communicating inner circumferential surface with the piston located at the rearmost position; and a seal-ring mounting groove is formed in the outer circumferential surface of the piston over the entire circumference, and the seal ring is mounted therein.