pins or shear screws 43 provide a releasable connection between the pressure cylinder 42 and the stepped portion 40 on the next intermediate stepped portion is a packing element 45 which provides a pressure seal between the sleeve 41 and the stepped portion 40 a stop nut 47 is threadedly received on the section 35 for retaining the packing element 45 in a fixed position the tubular sleeve 41 has a portion 50 of reduced diameter which is slidably received on the outer wall of the upper mandrel section 35 the reduced portion 50 has a packing element 51 disposed in sealing relationship with respect to the outer wall of the mandrel section 35 a housing 42a is threadedly attached to the pressure cylinder 42 in the housing 42a is a plunger recess 52 which has a spring biased pin member 53 arranged to resiliently bear against the outer wall of section 35 between the packing elements 45 and 51 the upper mandrel section 35 has a pressure port 54 through a wall which is adapted to pass fluid from the interior of the section 35 to the interior of the cylinder 42 to act upon the pressure cylinder 42 and to move the cylinder 42 in an upward direction upon the application of sufficient pressure to shear the pins 43 just below.