a pressurized gas preferably an inert gas such as nitrogen above a level of hydraulic fluid 40 which communicates through a conduit 42 in turn to conduits 44 and 46 conduit 44 allows the fluid 40 to exert a force against surface 30 of piston 10 the pressure in conduit 44 is communicated through passageway 32 to the area between seals 26 and 36 however the pressure thus communicated through passageway 32 does not act to operate piston 10 during normal operations in essence passageway 32 constitutes a pressure leak path to ensure that the control system c puts the sssv in a closed position if a failure occurs at seal 36 a secondary reservoir 48 communicates with a surface 50 of an equalizing piston 52 a seal 54 isolates secondary reservoir 48 from conduit 20 in the position shown in fig1 seal 56 in the position shown in fig1 isolates conduit 20 from conduit 46 between conduit 46 and piston 52 as shown in fig1 there is an enlarged bore 58 there 39 s also an enlarged bore 60 below seal 54 in the position shown in fig1 the purpose of the enlarged bores 58 and 60 is to permit bypass flow around the seals 54 and 56 after piston 52 shifts referring to fig3 when the equalizing piston 52 shifts due to failure of a variety of different components as will be explained below seal 56 no longer seals conduit 20 from conduit 46 thus allowing pressure from the.