the vicinity of a production formation 14 and which includes a double acting piston 15 fig2 and 3 which is reciprocable in a cylinder 17 to the respective ends of which a pair of fluid supply lines 19 and 21 are connected a production pipe 23 connects the pump 11 to a wellhead 25 and provides a conduit for the passage of production fluid from the production formation to the wellhead a four way valve 27 selectively connects the two supply lines 19 and 21 to a supply of fluid under pressure or to an exhaust line the four way valve 27 the supply of pressurized fluid 24 the main pressurized fluid inlet line 26 and the main pressurized fluid exhaust line 28 are of conventional construction and will not be further described turning now to fig2 and 3 it will be seen that the downhole pump 11 comprises an outer cylindrical casing 29 closed at its ends by upper and lower end caps 31 and 33 which also provide mountings for an inner cylindrical member 35 coaxially disposed within the outer casing 29 the inner cylindrical member 35 is divided into upper and lower halves 37 and 39 by a fixed bronze plug 41 which is secured in position by three radially extending flanges 43 extending through openings 45 in.