back to the reservoir if it is assumed that the oil pressure in the piston rod chamber is approximately zero the force on the piston rod equals the pressure in the cylinder times the piston area f pa the piston moves instead downwards if oil is pumped into the piston rod side chamber and the oil from the piston area flows back to the reservoir without pressure the pressure in the piston rod area chamber is pull force piston area u2212 piston rod area referring now to fig1 through 11 a repositioner 10 for a tractor truck 56 having a semi trailer 54 may include two hydraulic jack arms pivotally attached to the upper frame 12 the arms may each comprising a hydraulic cylinder 20 containing a movable piston the hydraulic cylinder 20 is generally a mechanical actuator that includes a movable piston that is used to give a linear force through a linear stroke one end of the piston is housed inside the cylinder and the other end is exposed to the outside for providing the linear stroke the upper frame 12 can be attached to an underside area of the semi trailer 54 at a location toward the end half of the semi trailer 54 opposing the tractor truck 56 for example at or toward a middle area between the middle of the semi trailer 54 and its end the upper frame 12 may be made from any metal or alloy suitable for the purpose of withstanding the weight and pressure exerted from a load of 60 tons or more the frame 12 may include any suitable configuration and support members for sturdy attachment to the bottom side of the semi trailer 54.