chamber 44 to the main chamber 40 to maintain the door 18 in the open position a counterbalancing force is provided to negate the restoring force of the spring assembly 42 on the main piston 38 by a counterbalancing assembly 46 mounted withn the secondary body 30 as may be best seen with particular reference to fig2 and 4 the counterbalancing assembly 46 includes a secondary piston 48 which is mounted within the secondary body 30 to sealably move therein and provide a chamber 50 which is either pressurized or vented depending upon the position of sealing valves 52 53 the secondary piston 48 is biased by a spring 49 which allows the secondary piston 48 to follow the main piston 38 the sealing valve 52 53 either blocks or unblocks a vent passageway 54 in response to the energized or unenergized state of a solenoid 56 to either block or open fluid flow from the chamber 50 to the main fluid chamber 40 through the porting 32 the sealing valve 52 includes a solenoid controlled valve member 82 which is moved to its closed position as is illustrated in fig4 upon energization of the solenoid 56 the valve member 82 controls the fluid flow from the chamber 50 to the main piston chamber 40 to this end a passageway 96 provides fluid communication between the chamber 50 and the valve member 82 the valve member 82 then controls fluid flow from the chamber 50 through the passageway 84 which communicates with passageways 86 and 54 the fluid flows from passageway 54 into an annular chamber 89 which communicates with a passageway 88 which hs one end connected to the selected porting passageway 32 the selected porting passageway 32 then acts to direct fluid therethrough from.