Vacuum booster diaphragm mounting

In an improved vacuum booster device comprising;

• a booster shell;
• a booster piston housed in said booster shell in a forwardly and backwardly movable manner;
• a diaphragm including an inner peripheral bead tilted onto a rear surface of said booster piston, an outer peripheral bead fitted onto the peripheral wall of said booster shell and a pressure receiving portion superposed on the surface of said booster piston, said diaphragm being adapted to divide the interior of said booster shell into a front-side first working chamber communicating with a vacuum source and a rear-side second working chamber in cooperation with said booster piston;
• an input rod supported by said booster piston for advancing and retracting movements;
• and a control valve operably connected with said input rod to selectively communicate said second working chamber with said first working chamber or the atmosphere, the improvement wherein said inner peripheral bead of said diaphragm is tightly fitted in an annular fitting groove formed in the rear surface of said booster piston with the aid of a clamping force given by a clamping band mounted to said inner peripheral bead;
• said inner peripheral bead having a bottom surface fitted to a bottom portion of said fitting groove, a side surface fitted to a rear side wall of the groove and an upper surface located opposite to said bottom surface receiving said clamping band, a part of said inner peripheral bead being formed as a recessed portion having a thin-walled side wall portion in close contact but separably with the rear surface of said booster piston and a thin-walled bottom wall portion in close contact but separably with the bottom portion of said fitting groove, said thin-walled side wall portion of the recessed portion being recessed from a side on which said recessed portion faces said clamping band so as to have a thickness smaller than that of the pressure receiving portion of said diaphragm, said recessed portion engaging with a notched recess formed in the rear side wall of said fitting groove;
• wherein said recessed portion constitutes a thin-walled elastic valve portion which is normally in close-contact with the rear surface of the booster piston and the bottom portion of said fitting groove and which is deformable as required to provide communication between said first and second working chambers through said valve portion free from engagement with said clamping band.