Relief valve utilizing back pressure

Claim 1

IN COMBINATION, A PAIR OF PILOT OPERATED PRESSURE RELIEF VALVES ADAPTED FOR CONNECTION IN AN OPPOSITE SENSE ACROSS THE LINES OF A HYDRAULIC TRANSMISSION; EACH OF SAID RELIEF VALVES INCLUDING A VALVE CHAMBER HAVING INLET AND OUTLET PORTS ADAPTED FOR CONNECTION TO SAID LINES, A PISTON FITTED IN SAID VALVE CHAMBER AND NORMALLY URGED TO A POSITION FOR CLOSING COMMUNICATION BETWEEN SAID PORTS, AN END OF SAID VALVE CHAMBER DEFINING A HOLD-DOWN CHAMBER IN WHICH FLUID UNDER PRESSURE ACTS ON AN END OF SAID PISTON FOR MAINTAINING THE VALVE CLOSED; SAID PAIR OF VALVES EACH HAVING ITS SAID INLET PORT CONNECTED TO THE SAID OUTLET PORT OF THE OTHER VALVE, AND UNIDIRECTIONAL VALVES CONNECTED, RESPECTIVELY, BETWEEN EACH OF THE INTERCONNECTED SAID VALVE PORTS AND THE HOLD-DOWN CHAMBERS SO AS TO ADMIT PRESSURE FLUID TO THE HOLD-DOWN CHAMBERS FROM THE ONE OF SAID LINES HAVING A HIGHER PRESSURE THAN EXISTS IN THE OTHER OF SAID LINES.