Brake servo-unit comprising a floating element bearing offset valve seats

A pneumatic servomotor (11) for power-assisted braking for a motor vehicle, which servomotor is:

• of the type having a rigid casing (12) inside which a moving transverse partition (14) is mounted to define in leaktight manner a front chamber (16) subjected to a first pressure (P1), and a back chamber (18) subjected to a second pressure (P2) varying between the first pressure (P1) and a pressure (Pa) that is greater than the first pressure (P1), which partition can drive an actuating rod (28) for actuating a master cylinder (13) associated with the servomotor (11) via a reaction disk (32), and is returned resiliently by first return means;
• of the type having a control rod (38) moving inside the casing (12) selectively as a function of an axial inlet force exerted forwards against a return force applied to the rod (38) by second return means;
• of the type in which the movements of the control rod (38) can determine opening and closing of at least one axial inlet valve (52) and which is interposed between a pressure source subjected to the pressure (Pa) greater than the first pressure (P1) and the back chamber (18), and at least one axial balancing valve (50) and which is interposed between the front chamber (16) and the back chamber (18), for actuating the above-described moving partition (14);
• and
• of the type in which a plunger (46) passing through the moving partition (14) and secured to the end of the control rod (38) can drive the actuating rod (28) of the master cylinder (13) directly, via the reaction disk (32);
• said servomotor being characterized in that it includes:
• a one-piece floating tubular element (76) interposed substantially radially between the plunger (46) and the casing (12), which element is mounted to move axially, is returned resiliently towards the moving partition (14) by third return means, and has axially offset transverse faces (78, 80) that are provided with first sealing elements (82, 84) for sealing the axial inlet and balancing valves (50, 52);
• a complementary transverse second sealing element (86) for sealing the inlet axial valve (52), which element is carried by a back end of the plunger (46);
• and
• a complementary transverse second sealing element (88) for sealing the balancing axial valve (50), which element is provided on at least a portion of the back face (90) of the moving partition (14);
• and in that the back end (45) of the plunger (46) is provided with a dish-shaped washer (49) which includes the complementary transverse second sealing element (86) for sealing the inlet axial valve (52), and which receives in abutment ends (41, 123) of two coaxial springs (40, 122), a first spring (40) of high stiffness whose other end (39) is in abutment against the moving partition (14), and which forms the second return means associated with the control rod (38), and a second spring (122) of low stiffness whose other end (121) is in abutment against a shoulder face of the tubular element (76), and which forms the third return means associated with said tubular element (76) and which enables said tubular element to be urged to close the balancing valve (50).