Drain back should not be a problem on any automatic trans, just for the reason suggested, parking on a hill. If the seals are leaking, they are bad, fluid level should not play any part in it. The fluid is all over the place when the vehicle is in motion. Now if it's running out the top vent tube, you might have a level problem, as the fluid gets aerated from all the moving parts.
You should be able to fill the transmission to the top without leakage. It won't like it and should never be done, but the seals should hold!
