Master cylinder

1. A master cylinder comprising:

a cylinder main body with a bottomed cylindrical shape having a discharge passage of a brake liquid and a supply passage in communication with a reservoir;

a piston movably disposed in the cylinder main body and forming a pressure chamber configured to supply a hydraulic pressure into the discharge passage between the cylinder main body and the piston; and

a seal member installed in a circumferential groove formed in the cylinder main body and capable of sealing between the supply passage and the pressure chamber,

wherein the seal member comprises:

a base section capable of abutting a circumferential wall of the circumferential groove near an opening section of the cylinder main body;

an inner circumferential lip section extending from the base section toward a bottom section of the cylinder main body and configured to come in sliding contact with an outer circumference of the piston;

an outer circumferential lip section extending from the base section toward the bottom section of the cylinder main body and configured to abut a bottom section of the circumferential groove; and

a center lip section extending from the base section toward the bottom section of the cylinder main body between the outer circumferential lip section and the inner circumferential lip section and having a distal end configured to abut the circumferential wall of the circumferential groove near the bottom section of the cylinder main body,

wherein the circumferential wall of the circumferential groove near the bottom section of the cylinder main body has an outer wall section extending from the bottom section of the circumferential groove in a radial direction of the cylinder main body, a stepped surface section extending from the outer wall section in a bottom direction of the cylinder main body, and an inner wall section having a flat surface section parallel to the radial direction of the cylinder main body and formed further inside in the radial direction of the cylinder main body than the stepped surface section, and

wherein the inner wall section is formed at a position at which a distal end section of the center lip section overlaps the stepped surface section or the inner wall section in the radial direction of the cylinder main body when the seal member moves toward the bottom section of the cylinder main body.

2. The master cylinder according to claim 1 , wherein the stepped surface section is formed to be inclined with respect to the axial direction of the cylinder main body.

3. The master cylinder according to claim 1 , wherein the flat surface section of the inner wall section is formed at a position at which the center lip section first abuts the flat surface section of the inner wall section when the seal member moves toward the bottom section of the cylinder main body.

4. The master cylinder according to claim 1 , wherein the center lip section is formed to protrude in the axial direction farther than the outer circumferential lip section and the inner circumferential lip section.

5. The master cylinder according to claim 3 , wherein the center lip section is configured to abut the stepped surface section along with a movement of the piston after abutting the flat surface section of the inner wall section.

6. The master cylinder according to claim 1 , wherein the stepped surface section is formed at a position at which the center lip section first abut to the stepped surface section when the seal member moves toward the bottom section of the cylinder main body.

7. The master cylinder according to claim 6 , wherein the center lip section is configured to abut the flat surface section of the inner wall section along with a movement of the piston after abutting the stepped surface section.

8. The master cylinder according to claim 1 , wherein the inner wall section is formed with a chamfering section at an inner end portion in the cylinder radial direction.

9. A master cylinder comprising:

a cylinder main body with a bottomed cylindrical shape, in which a piston is movably disposed, having a discharge passage of a brake liquid and a supply passage in communication with a reservoir; and

a seal member installed in a circumferential groove formed in the cylinder main body and capable of sealing between the supply passage and a pressure chamber formed in the cylinder main body,

wherein the seal member comprises:

a base section capable of abutting a circumferential wall of the circumferential groove near an opening section of the cylinder main body;

a plurality of lip sections respectively extending from both ends of the base section in a radial direction; and

a center lip section extending from the base section toward the bottom section of the cylinder main body between the plurality of lip sections and having a distal end configured to abut the circumferential wall of the circumferential groove near the bottom section of the cylinder main body,

wherein the circumferential wall of the circumferential groove near the bottom section of the cylinder main body comprises:

an outer wall section extending from the bottom section of the circumferential groove in the radial direction of the cylinder main body;

a stepped surface section extending from the outer wall section in a bottom direction of the cylinder main body and formed to be inclined with respect to an axial direction of the cylinder main body; and

an inner wall section having a flat surface section parallel to the radial direction of the cylinder main body and formed further inside in the radial direction of the cylinder main body than the stepped surface section, and

wherein the inner wall section is formed at a position at which a distal end section of the center lip section overlaps the stepped surface section or the inner wall section in the radial direction of the cylinder main body when the seal member moves toward the bottom section of the cylinder main body.

10. The master cylinder according to claim 9 , wherein the flat surface section of the inner wall section is formed at a position at which the center lip section first abuts the flat surface section of the inner wall section when the seal member moves toward the bottom section of the cylinder main body.

11. The master cylinder according to claim 10 , wherein the center lip section is configured to abut the stepped surface section along with a movement of the piston after abutting the flat surface section of the inner wall section.

12. The master cylinder according to claim 9 , wherein the stepped surface section is formed at a position at which the center lip section first abut to the stepped surface section when the seal member moves toward the bottom section of the cylinder main body.

13. The master cylinder according to claim 12 , wherein the center lip section is configured to abut the flat surface section of the inner wall section along with a movement of the piston after abutting the stepped surface section.

14. The master cylinder according to claim 9 , wherein the center lip section is formed to protrude in the axial direction farther than the plurality of lip sections.

15. The master cylinder according to claim 9 , wherein the inner wall section is formed with a chamfering section at an inner end portion in the cylinder radial direction.

16. A master cylinder comprising a cylinder main body with a bottomed cylindrical shape, in which a piston is movably disposed, having a discharge passage of a brake liquid and a supply passage in communication with a reservoir,

wherein the master cylinder is provided with a circumferential groove in which a seal member capable of sealing between the supply passage and a pressure chamber formed in the cylinder main body is installed,

wherein the seal member comprises:

a base section capable of abutting a circumferential wall of the circumferential groove near an opening section of the cylinder main body;

a plurality of lip sections respectively extending from both ends of the base section in a radial direction; and

a center lip section further protruding than the plurality of lip sections in an axial direction from the base section toward the bottom section of the cylinder main body between the plurality of lip sections and having a distal end configured to abut the circumferential wall of the circumferential groove near the bottom section of the cylinder main body,

wherein the circumferential wall of the circumferential groove near the bottom section of the cylinder main body comprises:

an outer wall section extending from the bottom section of the circumferential groove in the radial direction of the cylinder main body;

a stepped surface section extending from the outer wall section in a bottom direction of the cylinder main body and formed to be inclined with respect to an axial direction of the cylinder main body; and

an inner wall section having a flat surface section parallel to the radial direction of the cylinder main body and formed further inside in the radial direction of the cylinder main body than the stepped surface section, and

wherein the inner wall section is formed at a position at which a distal end section of the center lip section overlaps the stepped surface section or the inner wall section in the radial direction of the cylinder main body when the seal member moves toward the bottom section of the cylinder main body.

17. The master cylinder according to claim 16 , wherein the flat surface section of the inner wall section is formed at a position at which the center lip section first abuts the flat surface section of the inner wall section when the seal member moves toward the bottom section of the cylinder main body.

18. The master cylinder according to claim 17 , wherein the center lip section is configured to abut the stepped surface section along with a movement of the piston after abutting the flat surface section of the inner wall section.

19. The master cylinder according to claim 16 , wherein the stepped surface section is formed at a position at which the center lip section first abut to the stepped surface section when the seal member moves toward the bottom section of the cylinder main body.

20. The master cylinder according to claim 19 , wherein the center lip section is configured to abut the flat surface section of the inner wall section along with a movement of the piston after abutting the stepped surface section.