Update: You are correct. I just unhooked the rear (return) line on the transmission and ran the car in park, neutral, and in the forward gears just for fun, no fluid coming out whatsoever. I did get a dribble of fluid from the hose when I first unhooked it though. Question: would the fluid not be pumping from the return line if the level was too low, or should it pump fluid at any level?
Barely loosen neutral safety switch. Start the engine. Remove the neutral safety switch. Add fluid until it runs out the neutral safety switch hole. Test gear engagement. Report back.
