out conduit 76 to a flowline not shown piston 72 maintains slidable sealing contact with inner wall 84 of pump chamber 52 by means of annular seal 85 positioned within groove 83 formed in wall 84 so as to prevent co mingling of power fluid f and well liquid l or well gas g conduit 86 allows well fluids to flow from formation 14 through screen 32 through the wall of casing 24 through the wall of tubular member 53 and into pump chamber 52 when piston 72 is in its lowermost position depicted in fig1 annular seal 88 suitably mounted within groove 90 formed with the inner wall 84 of the pump chamber is positioned so as to contact cylindrical surface 92 of piston 72 as shown in fig2 after piston 72 begins upward movement from its lowermost position liquid gas interface 94 of fig1 rises as liquid and gas flow into chamber 52 gas passing through vent valve 54 conduit 44 annulus 30 to flowline 16 until vent valve 54 closes to initiate a pump cycle as described herein below tubular member 53 may be provided with inwardly displaced radial shoulder 96 for supporting end 97 of spring 76 against upward movement such that spring 76 can provide sufficient force to return piston 72 to its lowermost position against the pressure of the power fluid as in fig1 piston 72 may be formed with stem 98 having outwardly disposed shoulder 99 to act against lower end 100 of spring 76 so.