First, the check valve missing will cause no problem other than oil draining back and possibly causing hesitation of hydraulic function on start up. I have seen many missing ones over the years. It is easy to have one fallout when splitting the tractor and it falls out of the line.
Second, it is entirely possible the supply and return lines are reversed where they fit into the clutch housing ports. The check valve should be in the line closest to right side of tractor as you are sitting in seat.
Second, it is entirely possible the supply and return lines are reversed where they fit into the clutch housing ports. The check valve should be in the line closest to right side of tractor as you are sitting in seat.