| 摘要 |
In a master cylinder including: a cylinder body having a cylindrical bore and an opening made in the circumferential wall, for a relief port communicating with the interior of hydraulic reservoir, a piston inserted slidably into said cylindrical bore to form a hydraulic pressure generating chamber in said cylindrical bore; and a cup seal having an inner circumferential lip portion contacting with said piston, an outer circumferential lip portion slidably contacting with the inner circumferential wall of said cylindrical bore, and a base portion connecting said lip portions which are so arranged as to face to said hydraulic pressure generating chamber, whereby said cup seal cuts the communication between said hydraulic pressure generating chamber and said hydraulic reservoir through said relief port when the top end of said outer circumferential lip portions passes by said relief port, the improvement in which means for rotating said cup seal relative to said cylindrical bore with the movement of said piston is provided at any other portion than a portion of said cup seal facing to the inner wall of said cylindrical bore.
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