on centers which correspond to one half the spacing of the adjacent inlet ports associated with the sleeve of the valve sub 33 as will be more fully explained later on in this disclosure shoulder 57 formed on the release arm 51 39 abuttingly engages shoulder 58 formed on the mandrel when the mandrel has been cycled to its uppermost position respective to the main outer housing shoulder 59 formed on the mandrel abuttingly engages shoulder 60 formed on the housing when the mandrel has been cycled to its lowermost position respective to the housing fill plug 61 provides a means by which a compressible liquid can be introduced into the annular working space formed between the mandrel and the main housing so that all of the compressible fluid can be evacuated therefrom by utilizing prior art expedients in fig5 and 6 a power chamber 62 of annular construction reciprocatingly receives a power piston 63 in sealed relationship therewithin the power piston is suitablely sealed to the sidewalls of the annular power chamber 62 by the illustrated seal means the power piston 63 divides the power chamber into a compressible fluid chamber 64 as for example air at atmospheric pressure and a clean liquid chamber 65 as for example oil the power piston 63 is affixed to the mandrel 38 and a pressure differential effected across the opposed faces of piston 63 moves the mandrel respective to the main housing at least one drilled passageway 66 extends from chamber 65 through a metering orifice 67 and into an oil reservoir 68 piston 69 is reciprocatingly received within the annular oil reservoir 68 and has opposed faces one of which is subjected to the.