Master cylinder

1. A master cylinder comprising:

a cylinder main body that has a bottomed cylinder shape including a discharging channel for a brake fluid and a feeding channel communicating with a reservoir;

a piston that is movably provided inside the cylinder main body and forms a pressure chamber that supplies a hydraulic pressure to the discharging channel between the piston and the cylinder main body; and

a seal that is disposed inside a circumferential groove formed to be open on an inner circumferential surface of the cylinder main body, so that an inner circumference comes into sliding contact with the piston to define the feeding channel and an exterior of the cylinder main body,

wherein the seal has an inner circumferential lip portion which protrudes from a toric base portion and comes into sliding contact with an outer circumferential surface of the piston, and an outer circumferential lip portion which abuts the circumferential groove of the cylinder main body,

wherein the inner circumferential lip portion has a length in a piston axis direction longer than the outer circumferential lip portion,

wherein the circumferential groove has an inclined surface portion formed in a circumferential wall on the feeding channel side such that a groove width increases from a groove bottom portion toward a groove opening portion,

wherein the inclined surface portion is configured such that:

i) in a non-braking state: a tip portion of the inner circumferential lip portion does not contact the inclined surface portion; and

ii) in a braking state: the tip portion of the inner circumferential lip portion does contact the inclined surface portion, and

wherein, displacement of the inner circumferential lip portion upon braking is thereby regulated by the inclined surface portion contacting the tip portion of the inner circumferential lip portion.

2. The master cylinder according to claim 1 ,

wherein a length of the seal in the piston axis direction from a tip portion of the outer circumferential lip portion to an end portion of the base portion on a side opposite to the outer circumferential lip portion is substantially equal to the groove width of the groove bottom portion of the circumferential groove.

3. The master cylinder according to claim 1 ,

wherein a length of the seal in the piston axis direction from a tip portion of the inner circumferential lip portion to an end portion of the base portion on a side opposite to the inner circumferential lip portion is larger than the groove width of the circumferential groove on a groove bottom side.

4. The master cylinder according to claim 3 ,

wherein the tip portion of the inner circumferential lip portion of the seal is disposed in the vicinity of the inclined surface portion.

5. The master cylinder according to claim 1 ,

wherein the groove width of the circumferential groove is larger than a groove depth.

6. A master cylinder comprising:

a cylinder main body that has a bottomed cylinder shape including a discharging channel for a brake fluid and a feeding channel communicating with a reservoir;

a piston that is movably provided inside the cylinder main body and forms a pressure chamber that supplies a hydraulic pressure to the discharging channel between the piston and the cylinder main body; and

a seal that is disposed inside a circumferential groove formed to be open on an inner circumferential surface of the cylinder main body, so that an inner circumference comes into sliding contact with the piston to define the feeding channel and an exterior of the cylinder main body,

wherein the circumferential groove has a bottom side circumferential wall that includes an outer wall surface portion and an inner inclined surface portion,

wherein the seal has an inner circumferential lip portion which protrudes from a toric base portion and comes into sliding contact with an outer circumferential surface of the piston, and an outer circumferential lip portion which abuts the circumferential groove of the cylinder main body,

wherein a protrusion portion is provided on a front side of the inner circumferential lip portion,

wherein the outer circumferential lip portion includes a first tip portion, and the protrusion portion includes a second tip portion,

wherein the inner circumferential lip portion has a length in a piston axis direction longer than the outer circumferential lip portion,

wherein the outer wall surface portion and the inner inclined surface portion are configured such that:

i) in a non-braking state: only the first tip portion of the outer circumferential lip portion planarly contacts the outer wall surface portion, and

ii) in a braking state: the first tip portion of the outer circumferential lip portion planarly contacts the outer wall surface portion and at the same time, the second tip portion of the protrusion portion planarly contacts the inner inclined surface portion, and

wherein, displacement of the inner circumferential lip portion upon braking is thereby regulated by the second tip portion of the protrusion portion planarly contacting the inner inclined surface portion.

7. The master cylinder according to claim 6 ,

wherein a length of the seal in the piston axis direction from a tip portion of the outer circumferential lip portion to an end portion of the base portion on a side opposite to the outer circumferential lip portion is substantially equal to the groove width of the groove bottom portion of the circumferential groove.

8. The master cylinder according to claim 6 ,

wherein a length of the seal in the piston axis direction from a tip portion of the inner circumferential lip portion to an end portion of the base portion on a side opposite to the inner circumferential lip portion is larger than the groove width of the circumferential groove on a groove bottom side.

9. The master cylinder according to claim 8 ,

wherein the tip portion of the inner circumferential lip portion of the seal is disposed in the vicinity of the inclined surface portion.

10. The master cylinder according to claim 6 ,

wherein the groove width of the circumferential groove is larger than a groove depth.