Valve with positive shutoff

Claim 1

IN A VALVE HAVING A HOUSING ENCLOSING A VALVE SEAT INTERMEDIATE AN INLET AND AN OUTLET, THE COMBINATION OF: SEAL MEANS DEFINING A PRESSURE CHAMBER AND POSITIONED WITHIN THE HOUSING, SAID SEAL MEANS HAVING FIRST AND SECOND OPENINGS THEREIN WITH FLOW PATHS THERETHROUGH FOR FLUID COMMUNICATION WITH SAID PRESSURE CHAMBER, SAID SECOND OPENING BEING LOCATED IN A FLEXIBLE WALL OF SAID SEAL MEANS, SAID WALL BEING SO POSITIONED THAT WHEN FLEXED IN ONE DIRECTION IT WILL ENGAGE THE VALVE SEAT TO BLOCK FLOW OF FLUID BETWEEN THE INLET AND THE OUTLET, SAID FIRST OPENING CONNECTING SAID PRESSURE CHAMBER TO AN EXHAUST, SAID SECOND OPENING CONNECTING SAID PRESSURE CHAMBER WITH THE INLET; AND CONTROL MEANS ENGAGEABLE WITH SAID FIRST AND SECOND OPENINGS AND MOVABLE BETWEEN OFF AND ON POSITION FOR CLOSING SAID FIRST OPENING WHEN IN AN OFF POSITION, AND CLOSING SAID FIRST OPENING AND SAID SECOND OPENING WHEN IN AN ON POSITION, AND OPENING SAID FIRST OPENING WHEN IN A POSITION INTERMEDIATE SAID OFF AND ON POSITION, SAID CONTROL MEANS INCLUDING MEANS DEFINING A LEAKAGE PATH THROUGH SAID SECOND OPENING WHEN IN SAID ON POSITION, WHICH LEAKAGE PATH IS SMALL RELATIVE TO THE FLOW PATH THROUGH SAID SECOND OPENING WHEN IN SAID OFF POSITION.