a tubular outer housing 38 that defines a fluid chamber 40 therewithin fluid openings 42 are disposed through the housing 38 below the fluid chamber 40 a sliding sleeve 44 is retained within the housing 38 and includes a number of fluid ports 46 disposed radially therethrough seals 43 a and 43 b are disposed in outer housing 38 above and below fluid openings 42 when the sliding sleeve 44 is axially displaced such that piston 50 is near the bottom of chamber 40 the ports 46 are below lower seal 43 b and there is no flow into bore 48 of housing 38 depending upon the axial position of the sliding sleeve 44 within the housing 38 and within the seals 43 a b the ports 46 of the sleeve 44 can be selectively aligned with the fluid openings 42 in the housing 38 to permit varying degrees of fluid flow into the bore 48 of the housing 38 as the ports 46 overlap the openings 42 in varying amounts the sliding sleeve 44 also includes an enlarged outer piston portion 50 that resides within the chamber 40 and separates chamber 40 into an upper chamber 52 and a lower chamber 54 a seal not shown on the outer diameter of piston 50 hydraulically isolates upper chamber 52 and lower chamber 54 piston 50 exposes substantially equal piston area to each of chambers 52 and 54 such that equal pressures in chambers 52 and 54 result in substantially equal and opposite forces on piston 50 such that piston 50 is considered u201c balanced u201d to move piston 50 a.