15 a maybe substantially counteracted by the force exerted on the reversing spool 16 by the pressure balancing piston 18 with the position piston 17 exerting a force on the reversing spool 16 towards the second end 3 of the assembly 1 and pressing the reversing spool 16 against the reversing sleeve 22 the reversing sleeve 22 maybe pressed against a bumper 30 in the inner housing 5 when the first piston 7 a reaches the top of a power stroke the reversing sleeve 22 and the reversing spool 16 may act in conjunction to reverse the direction of motion of the first piston 7 a and the second piston 7 b referring to fig2 as the first piston 7 a reaches an end of the power stroke a bottom of the second piston 7 b may come into contact with the second end 19 b of the reversing spool 16 because of the balancing of the forces on the reversing spool by the balancing pressure piston 18 the second piston 7 b may only have to exert a force on the reversing spool 16 to overcome the force exerted on the reversing spool 16 towards the second piston 7 b by the position piston 17 with the first piston 7 a overcoming the force placed on the reversing spool 16 by the position piston 17 the reversing spool 16 maybe shifted by the second piston 7 b towards the first end 2 of the assembly 1 referring to fig3 with the reversing spool 16 shifted.