Patent ID: 7181911

Claim:
A plunger type master cylinder comprising a cylinder housing having a cylinder bore axially extending from a first end part which is open and a second end part which is closed and a communication passage which is communicated with a reservoir, a piston fitted into said cylinder bore of said cylinder housing, defining a fluid pressure chamber on the side of said second end part which is closed and axially movable, a seal ring located on the side of said cylinder housing, adapted to seal an outer periphery of said piston, and including an inner lip and an outer lip extending from a base part and an inner and an outer peripheral part of said base part to the side of said fluid pressure chamber, respectively, and a relief port serving as a passage which is located on the side of said piston and adapted to intercommunicate said communication passage and said fluid pressure chamber, an opening of said passage being located at an outer peripheral wall surface of said piston, said plunger type master cylinder having the following features; a recess is formed on an outer peripheral surface of said piston ranging, when axially viewed, at least from a second edge on the side of said second end part of said opening of said relief port to a first edge on the side of said first end part of said opening of said relief portion, further ranging to an area away by a predetermined distance from said first edge of said opening of said relief portion toward said first end part side, and further ranging over the entire length in a peripheral direction of said piston, said inner lip of said seal ring is smaller in diameter at a distal end part as a free end thereof than at a root part on said base part side, thereby said seal ring has a seal coupling part situated at an inner periphery of the distal end part of said inner lip which is small in diameter which is seal-coupled to an outer peripheral wall surface of said piston and adapted to seal-coupled to an outer peripheral wall surface of said piston, and moreover, a void for communicating said relief portion with said communication passage together with said recess is formed between an inner periphery of said inner lip ranging from said seal coupling part to said root part, and said seal coupling part of the distal end part of said inner lip of said seal ring is axially located at the opening part of said relief port on the side of said piston when said master cylinder is in a non-operating position.