PLUNGER TYPE MASTER CYLINDER

PROBLEM TO BE SOLVED: To avoid pressure containment in a hydraulic chamber in a state where a piston is at a non-operation position, in a plunger type master cylinder.SOLUTION: This plunger type master cylinder is structured such that a piston port 21a formed on a piston 21, opened to an outer peripheral wall surface of the piston 21 and communicating with a hydraulic chamber Rp1, and an annular seal point part 31b1 arranged in an inner periphery of a tip part of an inner peripheral lip part 31b of a seal ring 31 and seal-connectable to the outer peripheral wall surface of the piston 21 are set to overlap each other, and the hydraulic chamber Rp1 communicates with a reservoir through the piston port 21a and a communication passage. The inner peripheral lip part 31b of the seal ring 31 is provided with a plurality of axial communication grooves 31b2 extending from a base part-side end of the seal point part 31b1 toward a base part 31a, and a circumferential communication groove 31b3 for making seal point part-side ends of them circumferentially communicate with one another.