Vehicle braking system

1. A vehicle braking system comprising:

a master cylinder in which a master piston with its back turned to a boosted hydraulic pressure chamber is slidably housed in a casing;

a hydraulic booster which has a control piston for achieving a balance between reaction force generated by hydraulic pressure of the boosted hydraulic pressure chamber and brake operating input from a brake operating member, and which applies output hydraulic pressure of a hydraulic power source to the boosted hydraulic pressure chamber after regulating the output hydraulic pressure as the control piston moves axially; and

a brake stroke simulator installed between the brake operating member and the control piston in order for a driver to feel operating strokes of the brake operating member,

the master cylinder being connected to wheel brakes,

wherein the hydraulic booster includes a backup piston which is slidably housed in the casing with its front face turned to the boosted hydraulic pressure chamber and which can directly push the master piston in response to operation of the brake operating member when the hydraulic pressure of the boosted hydraulic pressure chamber decreases,

wherein the brake stroke simulator is contained in the control piston and the control piston is completely contained in the backup piston.

2. The vehicle braking system according to claim 1 , wherein the control piston is formed into a bottomed cylindrical shape with an end wall at a front end, wherein the brake stroke simulator comprises an input member linked to the brake operating member, a simulator piston which is slidably fitted in the control piston to form a stroke liquid chamber between itself and the end wall of the control piston and which is coupled to the input member, and elastic members installed between the input member and the control piston, and wherein a port which is closed when the control piston advances through a predetermined travel distance or more toward the backup piston is provided in the end wall of the control piston so as to communicate the stroke liquid chamber with a reservoir when the port is open.

3. The vehicle braking system according to claim 1 , wherein the backup piston includes an axially extending cylindrical body.

4. The vehicle braking system according to claim 1 , further comprising a control valve device for regulating pressure in the boosted hydraulic pressure chamber, wherein the control valve device is contained in the backup piston, and wherein the pressure introduced into the boosted hydraulic pressure chamber and regulated by the control valve device pushes the master piston of the master cylinder for operation.

5. The vehicle braking system according to claim 1 , wherein the brake stroke simulator is completely contained in the control piston.

6. The vehicle braking system according to claim 1 , wherein an open end of the backup piston faces the master piston.

7. A vehicle braking system comprising:

a master cylinder in which a master piston with its back turned to a boosted hydraulic pressure chamber is slidably housed in a casing;

a hydraulic booster which has a control piston for achieving a balance between reaction force generated by hydraulic pressure of the boosted hydraulic pressure chamber and brake operating input from a brake operating member, and which applies output hydraulic pressure of a hydraulic power source to the boosted hydraulic pressure chamber after regulating the output hydraulic pressure as the control piston moves axially;

a brake stroke simulator installed between the brake operating member and the control piston in order for a driver to feel operating strokes of the brake operating member,

the master cylinder being connected to wheel brakes,

wherein the hydraulic booster includes a backup piston which is slidably housed in the casing with its front face turned to the boosted hydraulic pressure chamber and which can directly push the master piston in response to operation of the brake operating member when the hydraulic pressure of the boosted hydraulic pressure chamber decreases, and

wherein the control piston and the brake stroke simulator are completely contained in the backup piston; and

a control valve device for regulating pressure in the boosted hydraulic pressure chamber,

wherein the control valve device is contained in the backup piston, and

wherein the pressure introduced into the boosted hydraulic pressure chamber and regulated by the control valve device pushes the master piston of the master cylinder for operation.

8. The vehicle braking system according to claim 7 , wherein the brake stroke simulator is contained in the control piston, wherein the control piston is formed into a bottomed cylindrical shape with an end wall at a front end, wherein the brake stroke simulator comprises an input member linked to the brake operating member, a simulator piston which is slidably fitted in the control piston to form a stroke liquid chamber between itself and the end wall of the control piston and which is coupled to the input member, and elastic members installed between the input member and the control piston, and wherein a port which is closed when the control piston advances through a predetermined travel distance or more toward the backup piston is provided in the end wall of the control piston so as to communicate the stroke liquid chamber with a reservoir when the port is open.

9. The vehicle braking system according to claim 7 , wherein the backup piston includes an axially extending cylindrical body.

10. The vehicle braking system according to claim 7 , wherein the brake stroke simulator is completely contained in the control piston.

11. The vehicle braking system according to claim 7 , wherein an open end of the backup piston faces the master piston.

12. A vehicle braking system comprising:

a master cylinder in which a master piston with its back turned to a boosted hydraulic pressure chamber is slidably housed in a casing;

a hydraulic booster which has a control piston for achieving a balance between reaction force generated by hydraulic pressure of the boosted hydraulic pressure chamber and brake operating input from a brake operating member, and which applies output hydraulic pressure of a hydraulic power source to the boosted hydraulic pressure chamber after regulating the output hydraulic pressure as the control piston moves axially; and

a brake stroke simulator installed between the brake operating member and the control piston in order for a driver to feel operating strokes of the brake operating member,

the master cylinder being connected to wheel brakes,

wherein the hydraulic booster includes a backup piston which is slidably housed in the casing with its front face turned to the boosted hydraulic pressure chamber and which can directly push the master piston in response to operation of the brake operating member when the hydraulic pressure of the boosted hydraulic pressure chamber decreases,

wherein the brake stroke simulator is completely contained in the control piston, and wherein the control piston is completely contained in the backup piston.

13. The vehicle braking system according to claim 12 , wherein the control piston is formed into a bottomed cylindrical shape with an end wall at a front end, wherein the brake stroke simulator comprises an input member linked to the brake operating member, a simulator piston which is slidably fitted in the control piston to form a stroke liquid chamber between itself and the end wall of the control piston and which is coupled to the input member, and elastic members installed between the input member and the control piston, and wherein a port which is closed when the control piston advances through a predetermined travel distance or more toward the backup piston is provided in the end wall of the control piston so as to communicate the stroke liquid chamber with a reservoir when the port is open.

14. The vehicle braking system according to claim 12 , wherein the backup piston includes an axially extending cylindrical body.

15. The vehicle braking system according to claim 1 , wherein an open end of the backup piston faces the master piston.