120b provided around the exterior of the first piston head 118 for sealing contacting engagements across smooth inner wall 102a of the primary piston housing member 102 as better illustrated in fig2 a 2d the secondary piston member housing 103 provides a stroking chamber 1120 therein for receipt of a substantially incompressible fluid 130 such as tap water which fills the stroking chamber 1120 from the bottom thereof to the approximate top defined by a fluid normal fill line 124b the top of the column of the substantially incompressible fluid 130 within the stroking chamber 1120 is defined by the normal fill line 124b and defines the lowermost end of a thermal expansion area 124a the top of the thermal expansion area 124a constitutes the effective inner face or second piston head 122a the second piston assembly 122 is secured at threads 121 to the lowermost end of a stroking rod 120 and has defined thereacross a second differential pressure area 123c immediate primary and secondary elastomeric seal members 123a and 123b housed within companion grooveways around the exterior of the second piston assembly 122 the second piston assembly housing 122 provides a control fluid chamber 124 therein which extends to the lowermost end of the secondary piston member housing 103 thus the outer housing 100 extends lowerly of the primary piston housing member 102 by means of the secondary piston member housing 103 and is terminated at the lowermost end by means of a cross over housing member 104 secured to the secondary piston member housing 103 by threads 105 seals 108a and 108b are provided to prevent fluid communication between the secondary piston member housing 103 and cross over housing member 104 the cross over housing member 104.