and the top of close actuator 28 and the lower part of close actuator 28 is vented to retain valve member 22 in its open position actuating fluid must be supplied continually to the open line of selector valve 34 and to regulator 32 as soon as the pressure in the open line to selector valve 34 is reduced the valve returns to its central position shown wherein both the close and open actuators are vented and the valve member 22 returns to closed position valve member 22 may be closed by supplying actuating fluid to pressure regulator 32 and to the close line leading to selector valve 34 this allows the valve 34 to be positioned to deliver actuating fluid from regulator 32 to close actuator 28 and at the same time vents open actuator 30 to assure that it does not resist closing actuation of close actuator 28 pulls the close cables 24 upwardly the upward movement of cables 24 raises the inner ends of close links 40 to cause the plate 48 of the mechanism 46 to be forced downwardly whereby valve member 22 is positively moved into seated position on the seat ring 42 with the valve member 22 in engagement with seat ring 42 any break in or separation of line 16 from tank 10 does not result in the spilling of fluids contained in tank 10 unless valve member 22 is being positively held in its open position by open actuator 30 it returns to seated position by having.