Safety shut-off for piston pump and valve unit

Claim 1

IN A VALVE UNIT HOUSING HAVING SUCTION AND DISCHARGE PASSAGEWAYS AT ONE END AND FLUID PRESSURE AND EXHAUST CONNECTIONS AT THE OTHER END, AND A VALVE ELEMENT BETWEEN SAID ENDS FOR SELECTIVELY CONNECTING THE SUCTION PASSAGEWAY WITH THE FLUID PRESSURE CINNECTION, SAID HOUSING HAVING A SUCTION DUCT CONNECTIED TO SAID SUCTION PASSAGEWAY AT AN ANGLE THERETO AND OPENING THROUGH THE HOUSING AT ONE END, AN ANNULAR SEAT IN THE OPPOSITE END OF SAID DUCT, A BALL CHECK NORMALLY SEATED ON SAID SEAT FOR OPENING AWAY FROM SAID SUCTION PASSAGEWAY, A PISTON MOVABLY MOUNTED IN THE OPEN END OF SAID DUCT AND HAVING AN EFFECTIVE AREA GREATER THAN THE EFFECTIVE AREA OF THE BALL CHECK WITHIN SAID SEAT, AN O-RING SEAL FOR THE PISTON, SAID PISTON HAVING AN ANNULAR GROOVE RECEIVING SAID ORING OF SUBSTANTIALLY GREATER AXIAL WIDTH THAN SAID RING IN SEALING POSITION, AND MEANS RIGIDLY CONNECTING SAID PISTON TO SAID BALL CHECK.