Master cylinder

A master cylinder (1) includes a cylinder body (2) having a cylindrical bore (3) and an opening made in the circumferential wall, for a relief port (13) communicating with the interior of a hydraulic reservoir, a piston (5) inserted slidably into the cylindrical bore (3) to form a hydraulic pressure generating chamber (7) in the cylindrical bore (3), and a cup seal (10) having an inner circumferential lip portion (32) contacting the piston (5), an outer circumferential lip portion (31) slidably contacting with the inner circumferential wall of the cylindrical bore (3), and a base portion (30) connecting the lip portions (31,32) which are so arranged as to face the hydraulic pressure generating chamber (7), whereby the cup seal (10) cuts communication between the hydraulic pressure generating chamber (7) and the hydraulic reservoir through the relief port (13) when the top end of the outer circumferential lip portion (31) passes by the relief port (13). The master cylinder (1) includes means for rotating the cup seal (10) relative to the cylindrical bore (3) with movement of the piston (5), at any portion other than a portion of the cup seal (10) facing the inner wall of the cylindrical bore (3).