Sealing member for master cylinder

1. A sealing member for a master cylinder, which is formed in a ring shape and is installed between an inner surface of a cylinder body and an outer surface of a piston, comprising:

an inner wing part configured to contact the outer surface of the piston;

an outer wing part configured to contact the inner surface of the cylinder body; and

a sealing rear wall part configured to connect the inner wing part and the outer wing part,

wherein the sealing rear wall part is provided with an oil passage to connect an oil entrance formed at the inner wing part and an oil entrance formed at the outer wing part, and the oil passage includes a retardation passage which is longer than a shortest straight-line distance which directly connects the oil entrances.

2. The sealing member for a master cylinder according to claim 1 , wherein the oil passage includes plural concentric passages formed in a concentric circle shape and plural radial passages connecting the concentric passages in a radial direction,

and wherein the radial passages are arranged in misalignment from each other.

3. The sealing member for a master cylinder according to claim 1 , wherein the oil passage is formed in a spiral shape.

4. The sealing member for a master cylinder according to claim 1 , wherein the oil passage is formed such that a flowing time of fluid that passes through the oil passage is longer than a time taken to form brake pressure in a hydraulic pressure chamber in braking operation.

5. The sealing member for a master cylinder according to claim 1 , wherein the retardation passage includes at least one of a circumferential passage formed in a circumferential direction and a slanted passage slanted with respect to the shortest straight-line distance.

6. The sealing member for a master cylinder according to claim 1 , wherein the retardation passage is longer than the shortest straight-line distance independent of pressure.

7. The sealing member for a master cylinder according to claim 1 , wherein the oil entrance formed at the inner wing part is circumferentially offset from the oil entrance formed at the outer wing part.

8. The sealing member for a master cylinder according to claim 1 , wherein the retardation passage includes a first and second circumferential passage and a radial passage connecting the first and second circumferential passage, the radial passage being circumferentially offset from at least one of the oil entrance formed at the inner wing part or the oil entrance formed at the outer wing part.