A ball valve assembly comprising: a body member extending from a first end to a second end and defining a body bore and a neck bore, the body bore having a central longitudinal axis; a first seat carrier housing a first seat, the first seat carrier disposed in the first end of the body member; a second seat carrier housing a second seat, the second seat carrier disposed in the second end of the body member; a ball member disposed in the body bore of the body member between the first and second seat carriers (40, 42), with the first and second seats (30, 32) engaging the ball member; a stem member disposed in the neck bore and coupled to the ball member; a seal retainer mounted to the second end of the body member, the seal retainer configured to retain the ball member, the first and second seat carriers (40, 42) and the first and second seats (30, 32) in the body member; wherein when the ball member is rotated to a fully closed position via the stem member, the ball member, the first and second seat carriers (40, 42) and the first and second seats (30, 32) are configured to move along the central longitudinal axis in a downstream direction in reaction to an upstream load or applied pressure, characterized in that a first gasketing material providing a first piston seal between an outer surface of the second seat carrier and an inner surface of the body bore of the body member; and a second gasketing material providing a second piston seal between the outer surface of the second seat carrier and an inner surface of the seal retainer; wherein the outer surface of the second seat carrier includes first and second grooves (23, 24), with the first gasketing material positioned within the first groove of the outer surface of the second seat carrier providing the first piston seal between the outer surface of the second seat carrier and the inner surface of the body bore of the body member, and with the second gasketing material positioned within the second groove of the outer surface of the second seat carrier providing the second piston seal between the outer surface of the second seat carrier and an inner surface of the seal retainer; wherein the first piston seal and the second piston seal prevent bypass between the body member and the seal retainer. a body member extending from a first end to a second end and defining a body bore and a neck bore, the body bore having a central longitudinal axis; a first seat carrier housing a first seat, the first seat carrier disposed in the first end of the body member; a second seat carrier housing a second seat, the second seat carrier disposed in the second end of the body member; a ball member disposed in the body bore of the body member between the first and second seat carriers (40, 42), with the first and second seats (30, 32) engaging the ball member; a stem member disposed in the neck bore and coupled to the ball member; a seal retainer mounted to the second end of the body member, the seal retainer configured to retain the ball member, the first and second seat carriers (40, 42) and the first and second seats (30, 32) in the body member; wherein when the ball member is rotated to a fully closed position via the stem member, the ball member, the first and second seat carriers (40, 42) and the first and second seats (30, 32) are configured to move along the central longitudinal axis in a downstream direction in reaction to an upstream load or applied pressure, characterized in that a first gasketing material providing a first piston seal between an outer surface of the second seat carrier and an inner surface of the body bore of the body member; and a second gasketing material providing a second piston seal between the outer surface of the second seat carrier and an inner surface of the seal retainer; wherein the outer surface of the second seat carrier includes first and second grooves (23, 24), with the first gasketing material positioned within the first groove of the outer surface of the second seat carrier providing the first piston seal between the outer surface of the second seat carrier and the inner surface of the body bore of the body member, and with the second gasketing material positioned within the second groove of the outer surface of the second seat carrier providing the second piston seal between the outer surface of the second seat carrier and an inner surface of the seal retainer; a first gasketing material providing a first piston seal between an outer surface of the second seat carrier and an inner surface of the body bore of the body member; and a second gasketing material providing a second piston seal between the outer surface of the second seat carrier and an inner surface of the seal retainer; wherein the outer surface of the second seat carrier includes first and second grooves (23, 24), with the first gasketing material positioned within the first groove of the outer surface of the second seat carrier providing the first piston seal between the outer surface of the second seat carrier and the inner surface of the body bore of the body member, and with the second gasketing material positioned within the second groove of the outer surface of the second seat carrier providing the second piston seal between the outer surface of the second seat carrier and an inner surface of the seal retainer; wherein the first piston seal and the second piston seal prevent bypass between the body member and the seal retainer.