| 摘要 |
A pneumatic brake servo comprises a casing (10) enclosing a piston (20) composed of a boss (34) and skirt (32). The casing (10) defines a front chamber (16) permanently connected to a vacuum source and a rear chamber (18) selectively connected to the front chamber (16) or to the atmosphere by means of a valve member (60, 62) actuated by a control rod (54) capable of coming to rest, through the front face of a plunger (44), on the rear surface of a reaction disc (48) integral with a thrust rod (52). A return spring (84) for the control rod (54) is disposed between the piston (34) and the plunger (44), the valve member (60, 62) including a shutter co-operating with a first circular valve seat (44a) on the piston (20), the shutter (60, 62) including a flexible membrane (60) of a generally truncated cone shape, sealingly fitted by its outer peripheral edge to the piston (20). According to the invention, the flexible membrane (60) is sealingly connected by its inner peripheral edge to a rigid tubular part (62) sealingly sliding in a rear tubular part (22) of the piston (20).
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