Pneumatic servo unit, method for making same and device

A pneumatic servomotor for an assisted braking, comprising a casing (2) having a longitudinal axis (X) and in which a low-pressure chamber (10) and a variable-pressure chamber (8) are defined, in an airtight manner, by a moving partition wall (12) bearing a pneumatic piston (18) in its middle part, a three-way valve (20), arranged at the rear part of the pneumatic piston (18) and controlled by an actuating rod (27), the three-way valve comprising a first valve (22) for a re-equalization between the low-pressure chamber (10) and the variable-pressure chamber (8), and a second valve (24) for the high-pressure supply of the variable-pressure chamber (8), said supply valve (24) comprising a valve seat, borne by a first longitudinal end of a distributor plunger (28) and receiving a longitudinal end of the actuating rod (27), said distributor plunger (28) having, at a second longitudinal in the opposite direction relative to the first longitudinal end, a finger (30), which is perpendicular to the longitudinal axis (X) and which comes into contact with a face of a reaction disk (32) on a braking operation, said reaction disk (32) being accommodated in a housing (34) integral with a push rod (36) for the actuation of a master cylinder, said housing (34) comprising a pneumatic braking-assistance device (D) fitted with a cage (38), a first closed longitudinal end of which is integral with the push rod (36) while its second open longitudinal end is closable by an annular ring (44) integral with the cage (38) and by a reaction piston (42) fitted for a sliding travel inside the ring (44) along the longitudinal axis (X), said reaction piston (42) being reactionnally kept against the ring (44) by a resilient means (40) having a prestress (ch) and bearing by its rear end on the closed first end of the cage (38), and wherein the an area consisting of the rear faces (50, 52) of the annular ring (44) and of the reaction piston (42), respectively, constitutes a front longitudinal end of the housing (34) of the reaction disk, characterised in that said device comprises plastically-deformable means (68, 74) for the adjustment of the braking characteristics.