{HEAD ~ Annulus pressure} then acts against the bottom side of piston 40 and moves {TAIL ~ the mandrel} up and opens the valve 
{ HEAD ~ Annulus pressure } then acts against the bottom side of piston 40 and moves { TAIL ~ the mandrel } up and opens the valve
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH