| 摘要 |
1. Vacuum brake-booster, for vehicles 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 sprint (19) towards the rear wall of the housing and comprising a hub (5) enclosing a distribution valve means (6) having a first valve seat (13) formed by the hub, a second valve seat (14) coaxial relative 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 relative to the rod (7), and a plunger-positioning member (22 ; 22') arranged in a radial recess (21) of the hub and having an inner end zone (24) extending in the bore (16) of the hub (15) and forming a stop surface (26, 32) cooperating selectively in axial bearing engagement with a shoulder (25) of the plunger, the positioning member (22 ; 22'), when the piston structure returns into a rest position in the vicinity of the rear wall (2) of the housing, being placed in a frist position which, by means of its stop surface (26), limits the retraction of the plunger (15) caused by the rod return spring (18), characterized in that the inner end zone of the positioning member (22 ; 22') has a first (26) and a second (32) stop surface which are offset axially and radially relative to one another, and in that the positioning member (22 ; 22') is radially displaceable in the recess (21), as a function of the axial displacement of the piston structure, between en first and a second position, in which the first (26) and the second (32) stop surfaces are respectively located axially opposite the shoulder (25) of the plunger (15). |
