| 摘要 |
In a negative pressure boosting device 1 of the present invention, a movable valve seat 28 is air-tightly and slidably disposed in a concavity of a valve body 4. The movable valve seat 28 and a valve element 12 compose a vacuum valve 15. The movable valve seat 28 and a control spring 29 compressed and disposed between the movable valve seat 28 and the valve body 4 compose a servo ratio changing means. The servo ratio changing means is provided on a valve mechanism (the vacuum valve 15). During operation, the valve element 12 comes in contact with the movable valve seat 28. However, the movable valve seat 28 does not move in a low-input range so that the servo ratio in this state is relatively large ratio i.e. the servo ratio for the normal operation. In the high-input range, the control spring 29 starts to be compressed to allow the movement of the movable valve seat 28, thereby changing the servo ratio to a small servo ratio. In this manner, the improvement of "controllability of brakes" in a low-input range and the improvement of "stiff feeling" in a high-input range are compatible.
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