Master cylinders for hydraulic transmission systems

Claim 1

IN A HYDRAULIC SYSTEM, THE COMBINATION OF A CYLINDER, A PISTON SLIDABLE IN SAID CYLINDER AND DEFINING THEREIN A COMPRESSION CHAMBER CONNECTED WITH UTILIZING CIRCUITS, SAID CYLINDER HAVING AN OPENING CONNECTED WITH A FLUID RESERVOIR, SAID PISTON HAVING A HEAD PORTION ON THE PISTON AND FACING SAID COMPRESSION CHAMBER, A PAIR OF AXIALLY SPACED ANNULAR RADIAL SHOULDERS ON SAID PISTON HEAD PORTION, SAID SHOULDERS FACING EACH OTHER AND DEFINING BETWEEN THEM A PERIPHERAL ANNULAR GROOVE IN SAID PISTON HEAD PORTION, SAID GROOVE HAVING A BOTTOM WITH OPENINGS THEREIN COMMUNICATING WITH SAID COMPRESSION CHAMBER, AN ANNULAR PACKING OF RESILIENT MATERIAL IN SAID GROOVE, SAID PACKING CONTACTING THE INNER SURFACE OF SAID CYLINDER AND BEING SPACED FROM THE BOTTOM OF SAID GROOVE, A RING OF RECTANGULAR CROSS SECTION WITH GREATER RADIAL DIMENSION THAN AXIAL THICKNESS DISPOSED IN SAID GROOVE BETWEEN SAID PACKING AND SAID RADIAL SHOULDER REMOTE FROM SAID COMPRESSION CHAMBER, SAID RING BEING OF MATERIAL THAT IS SUBSTANTIALLY UNDERFORMABLE AT PRESSURES OCCURRING IN SAID COMPRESSION CHAMBER AND SLIDABLY ENGAGING THE INNER SURFACE OF SAID CYLINDER WITHOUT CIRCUMFERENTIAL CLEARANCE WHILE BEING SPACED FROM THE BOTTOM OF SAID GROOVE, THE COMBINED THICKNESS OF SAID PACKING AND RING IN AN AXIAL DIRECTION BEING LESS THAN THE SPACING BETWEEN SAID SHOULDERS WHEREBY SAID PACKING AND RING ARE MOVABLE AXIALLY IN SAID GROOVE TO ACT AS A VALVE CONTROLLING COMMUNICATION BETWEEN SAID COMPRESSION CHAMBER AND SAID RESERVOIR THROUGH SAID OPENINGS IN THE BOTTOM OF SAID GROOVE.