The old underbody hoists are simple closed loop systems. The pump pulls oil from the top of the cylinder and pumps it into the bottom to extend the shaft. When the piston gets to the top it bypasses into the pickup or suction side of the pump. There should be an air vent built into the fill plug. If yours doesn't have one just use a pipe plug and drill a 1/8" hole thru it and put a cotter pin in the hole to offer some protection from dirt entering.
