Unless I misunderstand this, I'm not sure the return line is always uppermost. Think of the cooling system. The radiator, through which coolant is circulated, is outside the engine ("the mechanism"); nevertheless, the return line (coolant going back to the engine) is at the bottom, as it is on virtually all cars.The return line will always be the uppermost connection on any type vehicle where fluid is circulated outside the mechanism for cooling.
I have used this method successfully on my Toyota and Lexus cars. Did you shift through the gears while you were pumping out old fluid?I agree with plain OM, when I changed out my AT fluid, I disconnected the OUT hose from the oil filter and ran that into a gallon jug. For the adding of new fluid, I had a funnel in the trans filler tube (dipstick tube) and just kept pouring in the same amount of half gallons that were coming out. It worked perfectly.