| 摘要 |
In a negative pressure booster ( 1 ) of the present invention, at the time of performing a usual braking operation in a low deceleration region, a force attributed to the pressure difference between pressures applied to a variable pressure chamber and a constant pressure chamber is equal to or smaller than a sum of a set spring load of a spring ( 31 ) and a spring load of a valve spring ( 18 ). Accordingly, a vacuum valve seat member ( 27 ) is not moved and the usual braking operation is performed at a small servo ratio. Further, during the usual braking operation in an intermediate deceleration region, the force attributed to the pressure difference is larger than the sum of the above-mentioned spring loads and hence, the vacuum valve seat member ( 27 ) moves rearwardly while pushing a valve element ( 12 ). Accordingly, a valve opening quantity of an atmospheric valve ( 16 ) is increased while shortening a pedal stroke and hence, a braking operation for intermediate deceleration is performed at a larger servo ratio, whereby brake manipulation feeling can be enhanced.
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