1. A VALVE, COMPRISING: (A) A VALVE BODY STRUCTURE INCLUDING A FIXED ANNULAR VALVE SEAT, A CONFRONTING COAXIALLY DISPOSED VALVE CYLINDER, AN INLET PORT UPSTEAM FROM SAID VALVE SEAT AND AN OUTLET PORT DOWNSTREAM FROM SAID VALVE SEAT SAID VALVE CYLINDER HAVING A BLEED PORT AT ITS END REMOTE FROM SAID FIXED VALVE SEAT; (B) A PISTON VALVE SLIDABLE IN SAID CYLINDER AND ENGAGABLE WITH SAID FIXED VALVE SEAT, SAID PISTON VALVE FORMING WITH SAID CYLINDER A PRESSURE CHAMBER OF GREATER AREA THAN SAID FIXED VALVE SEAT, AND HAVING A METER PORT OF SMALLER DIAMETER EXTEDNING THERETHROUGH TO RECEIVE FLUID FROM UPSTREAM OF SAID FIXED VALVE SEAT; (C) A METER PIN ANCHORED IN SAID CYLINDER AND EXTENDING THROUGH SAID METER PORT TO FORM THEREWITH A PASSAGE SMALLER THAN SAID BLEED PORT; (D) AND MEANS FOR CAUSING SAID PISTON VALVE TO RECIPROCATE BETWEEN ITS FULLY OPEN POSITION AND A NEAROPEN POSITION TO MAINTAIN A WIPING ACTION BETWEEN SAID METER PORT AND SAID METER PIN.