| 摘要 |
1. Vacuum brake-booster, comprising : a housing having a rear wall (2) and divided internally into a vacuum chamber (3) and a working chamber (4) by a piston structure biased by a piston spring (19) towards the rear wall of the housing and comprising a hub a first valve seat (13) formed by the hub, a second valve seat (14) coaxial to the first valve seat and formed by the end of a valve plunger (15) sliding in a bore (16) of the hub and coupled to an actuating rod (7) of the booster, a valve member (11) mounted in the hub and biased axially by a valve spring (12) towards the first (13) and second (14) valve seats, to cooperate with the latter, a rod return spring (18) arranged in the hub coaxially to said rod (7), and a plunger-positioning member (22 ; 22') arranged in a radial recess (21) in the bub and having an inner end zone (25) extending into the bore (16) of the plunger (15) and forming a stop surface (23) cooperating selectively in axial bearing engagement with a shoulder (26) of the plunger, the positioning member (22 ; 22'), when the piston structure returns into its rest position in the vicinity of the rear wall (2) of the housing, being placed in a first position which, by means of its stop surface (23), limits the retraction of the plunger (15) under the effect of the rod return spring (18), characterized in that the positioning member (22, 22') has, axially opposite the stop surface (23), at least one bearing surface (27), the positioning member being radially displaceable in the recess (21), as a function of the axial displacement of the piston structure, between the first and the second positions, in which the bearing surface (27) is respectively up against a first (32) and a second (33) counterbearing surface which are offset axially and radially relative to one another in the hub. |
