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