| 摘要 |
A negative booster in which, when it is operated by an input not more than a preset level, a vacuum vale seat member (30) does not move and a reaction disc (25) is projected to be in contact with only a small diameter section (24a) of a spacing member (24). This causes a servo ratio to be a normal servo ratio. Also, when the negative booster is operated with an input exceeding the preset level, the vacuum valve seat member (30) is moved rearward (input side) relative to a valve body (4) by the pressure of a pressure changing chamber (9). This causes the reaction disc (25) to project greatly to be in contact with a large diameter section (24b) in addition to the small diameter section (24a). By this, a balance position at which both a vacuum valve (15) and an atmosphere valve (16) close moves rearward to increase a servo ratio. However, because the pressure receiving area of the spacing member (24) is increased to reduce the servo ratio, the servo ratio is the same as that when an input is not more than the set level.
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