| 摘要 |
PURPOSE: To impart sufficient braking force by communicating a second hydraulic pressure chamber with the master cylinder side by a fluid passage switching means when the internal pressure of a booster becomes abnormal pressure, thereby boosting hydraulic pressure from the master cylinder by a piston by the part of the pressure receiving area difference between the first and second hydraulic pressure chambers. CONSTITUTION: When the chamber A side is put in an abnormal pressure state due to the failure of a booster 2, a switching valve 44 is opened by a switching control part 51, and a hydraulic pressure chamber 39 is communicated with the communicating passage 34G side. When a brake pedal 1 is depressed in this state, the stepped piston 37 of a hydraulic pressure control valve 33 is slidingly displaced to the left since one end side is a small diameter part 37A so as to be small in area that receives pressure from a hydraulic pressure chamber 38 while a large diameter part 37B on the other end side is large in area that receives pressure from a hydraulic pressure chamber 39. When the supply-discharge port 34E and the hydraulic pressure chamber 38 are cut off from each other by a lip seal 59 or the like, it is put in the state communicating only with a brake pipeline 32 (the braking device 12, 14) side so as to impart sufficient braking force to each wheel. |
