Hitachi, Ltd.

Daigoro Kamase

Naganori Koshimizu

Tomonori Mouri

Yasuhiko Amari

A piston seal is provided in a circumferential groove formed in a cylinder body. The piston seal is capable of sealing a pressure chamber, which supplies fluid pressure to a discharge passage, from a supply passage, which communicates with a reservoir. The piston seal comprises: an annular base portion; an inner circumferential lip portion projecting from an inner circumferential side of the base portion and being in slidable contact with an outer circumferential surface of a piston; an outer circumferential lip portion projecting from an outer circumferential side of the base portion and being in abutment with the circumferential groove of the cylinder body; and an intermediate projecting portion which is formed between the inner circumferential lip portion and the outer circumferential lip portion of the base portion and which is projected beyond the outer circumferential lip portion. Cutout grooves are formed on a tip side of the intermediate projecting portion. Therefore, when releasing air, a sufficient amount of brake fluid can be supplied through a gap between the piston seal and a bottom surface of the circumferential groove, and the cutout groove. As a result, the number of repetitions of an air releasing operation can be minimized.

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Fukuoka, Japan

Daigoro Kamase