The top of a 39 is not even or square on all sides. front and back are completely different than the sides. Closed loop system will work as long as your pump still has suction power (not all dried out and seizing. Not hard to figure out . Turn it on and listen for the check valves and piston popping. With it running place the palm of your hand in the bottom of the suction pipe and feel for suction. If the pump is too far gone and not pumping then it will need rebuilding by a pro.

Rob