Patent ID: 7386979

Claim:
A plunger type master cylinder comprising: a cylinder body; a piston mounted in said cylinder body and having piston ports radially extending through a peripheral wall thereof, said piston defining a pressure chamber in said cylinder body; a primary cup fixed to said cylinder body and pressed against said piston; a reservoir communicating with a communication passage defined around said piston; said piston ports being positioned such that when said piston is in its inoperative position, said piston ports are not closed by said primary cup and said pressure chamber communicates with said communication passage through said piston ports; said piston having an inner periphery in which an annular groove is formed in its area where said piston ports are formed; an elastically deformable arcuate plate received in said annular groove; and a sleeve inserted in said arcuate plate and mounted to the inner periphery of said piston, thereby retaining said arcuate plate in said annular groove; said arcuate plate defining a gap as a fluid passage in cooperation with an inner surface of said annular groove while said arcuate plate is not elastically deformed; said pressure chamber communicating with said communication passage through cutouts or holes formed in said arcuate plate, holes formed in said sleeve, and said piston ports while said arcuate plate is not elastically deformed and said piston ports are not closed by said primary cup; said arcuate plate being configured to elastically and radially expand and close some of said piston ports when a pressure in said pressure chamber is higher by a predetermined value than a pressure in said communication passage.