Hi Steve B., with respect to your response about drain hole diameter ...
What is the size of your oil inlet hose?
Did you experiment with oil inlet size - if oil won't all come OUT perhaps we are putting too much IN?
Just a thought.
Jeff

Finn,

When I installed a vent in the top of my RD-1C drive it acted as an additional drain.  The drive operated completely filled with oil until the size of the original rear drain was increased to an ID of ~5/8" (1/2" NPT pipe).

Steve Boese