A valve assembly comprising: a valve body having a valve bore; a valve member slidably disposed within the valve bore between first and second stop positions; first and second radially extending valve elements (76, 78) connected to the valve member having first and second sealing surfaces (80, 82) oriented at equal angles with respect to a longitudinal axis of the valve member, wherein the first and second sealing surfaces are oriented substantially parallel with conical shaped first and second seat surfaces (112, 114) positioned in the valve body, wherein both of the first and second seat surfaces are oriented at equal angles with respect to the longitudinal axis and have an adjustable spacing such that the first sealing surface contacts the first seat surface and the second sealing surface contacts the second seat surface simultaneously in the valve member first stop position, wherein the first valve element includes a third sealing surface and the second valve element includes a fourth sealing surface, wherein the third and fourth sealing surfaces define mirror image orientations with respect to the first and second sealing surfaces (80, 82), wherein the third and fourth sealing surfaces are oriented parallel to each other and are each oriented at substantially equal angles with respect to the longitudinal axis of the valve member; conical shaped third and fourth seat surfaces (120, 122) each adapted to be contacted by one of the third and fourth sealing surfaces (116, 118), wherein both of the third and fourth seat surfaces are oriented at equal angles with respect to the longitudinal axis and are oriented parallel to the third and fourth sealing surfaces of the valve elements (76, 78) such that the third sealing surface contacts the third seat surface simultaneously with the fourth sealing surface contacting the fourth seat surface in a second stop position of the valve member; first and second adapters (88, 92) each selectively co-axially positioned with respect to the longitudinal axis and each including one of the first and second seat surfaces (112, 114); and first and second pistons (100, 102) positioned at opposite ends of the valve member each having only a single bore seal (108, 110) in each of the first and second pistons adapted to create a sliding seal. a valve body having a valve bore; a valve member slidably disposed within the valve bore between first and second stop positions; first and second radially extending valve elements (76, 78) connected to the valve member having first and second sealing surfaces (80, 82) oriented at equal angles with respect to a longitudinal axis of the valve member, wherein the first and second sealing surfaces are oriented substantially parallel with conical shaped first and second seat surfaces (112, 114) positioned in the valve body, wherein both of the first and second seat surfaces are oriented at equal angles with respect to the longitudinal axis and have an adjustable spacing such that the first sealing surface contacts the first seat surface and the second sealing surface contacts the second seat surface simultaneously in the valve member first stop position, wherein the first valve element includes a third sealing surface and the second valve element includes a fourth sealing surface, wherein the third and fourth sealing surfaces define mirror image orientations with respect to the first and second sealing surfaces (80, 82), wherein the third and fourth sealing surfaces are oriented parallel to each other and are each oriented at substantially equal angles with respect to the longitudinal axis of the valve member; conical shaped third and fourth seat surfaces (120, 122) each adapted to be contacted by one of the third and fourth sealing surfaces (116, 118), wherein both of the third and fourth seat surfaces are oriented at equal angles with respect to the longitudinal axis and are oriented parallel to the third and fourth sealing surfaces of the valve elements (76, 78) such that the third sealing surface contacts the third seat surface simultaneously with the fourth sealing surface contacting the fourth seat surface in a second stop position of the valve member; first and second adapters (88, 92) each selectively co-axially positioned with respect to the longitudinal axis and each including one of the first and second seat surfaces (112, 114); and first and second pistons (100, 102) positioned at opposite ends of the valve member each having only a single bore seal (108, 110) in each of the first and second pistons adapted to create a sliding seal.