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