Taper cup back piston

Claim 1

IN A SLUSH PUMP, A CYLINDER, A PISTON ADAPTED TO RECIPROCATE IN SAID CYLINDER, SAID PISTON INCLUDING A CYLINDRICAL METAL BODY HAVING AN OUTWARDLY EXTENDING MEDIAL FLANGE THEREAROUND OF SMALLER DIAMETER THAN THE INNER DIAMETER OF SAID CYLINDER TO AVOID CONTACT THEREBETWEEN, ANNULAR RESILIENT SEAL MEMBERS SLEEVED OVER EACH END OF SAID BODY IN ABUTTING RELATIONSHIP WITH SAID FLANGE, AN ANNULAR GROOVE IN EACH SEAL MEMBER TERMINATING IN THE OUTER CIRCUMFERENTIAL SURFACE AND IN THE FLANGE-ABUTTING SURFACE OF EACH SEAL MEMBER, NYLON BACK-UP RINGS DISPOSED IN SAID GROOVES TO CONSTRAIN SAID FLANGE-ABUTTING EDGE OF SAID SEAL MEMBERS FROM FLOWING OVER SAID FLANGE UNDER PRESSURE, THE RELAXED OUTER DIAMETER OF SAID RINGS BEING LESS THAN THE INNER DIAMETER OF SAID CYLINDER, THE RELAXED OUTER DIAMETER OF SAID SEAL MEMBERS BEING SUBSTANTIALLY EQUAL TO THE OUTER DIAMETER OF SAID RINGS BEGINNING AT SAID GROOVES AND TAPERING OUTWARDLY THEREFROM INTO WIPING CONTACT WITH SAID CYLINDER AT THE FREE ENDS OF SAID SEAL MEMBERS, SAID RINGS AND SEAL MEMBERS EACH HAVING AN ANNULAR INTERFACE AT LEAST A PORTION OF WHICH IS INCLINED OUTWARDLY WITH RESPECT TO THE AXIS OF SAID PISTON IN A DIRECTION AWAY FROM SAID FLANGE, THE RADIAL THICKNESS OF SAID RING ADJACENT ITS END REMOTE FROM SAID FLANGE AND THE ANGLE OF SAID INCLINED PORTION OF SAID INTERFACE BEING SUCH THAT THE COMPONENT OF THE RADIAL FORCE NORMAL TO SAID INTERFACE WILL CAUSE SUFFICIENT FRICTIONAL RESTRAINT TO MOVEMENT BETWEEN SAID INTERFACE SURFACES TO PROVIDE AN OUTWARD VELOCITY OF SAID INTERFACE SURFACE OF SAID SEAL UNDER THE INFLUENCE OF SAID RADIAL FORCE EQUAL TO OR LESS THAN THE RATE OF RADIAL EXPANSION OF SAID INTERFACE SURFACE OF SAID RING UNDER THE SAME RADIAL FORCE, WHEREBY SAID NYLON RING PROVIDES A TEAR-RESISTANT AND GRIT-EMBEDDING BACK-UP RING TO PREVENT FLOW OF SAID SEAL AROUND SAID FLANGE AND SAID RADIAL THICKNESS AND THE INCLINATION OF SAID INTERFACE PROVIDES EXPANSION OF SAID RING AT LEAST AS FAST AS SAID SEAL TO PREVENT FLOW OF SAID SEAL AROUND SAID RING, AND REMOVABLE MEANS ON EACH END OF SAID BODY TO RETAIN SAID SEALS THEREOVER.