[wolfdog]

So it seems near impossible to get unimpeded flow into the pan at 10qts engine off (10qts is with filter filled but that is negligible).

And if the check ball floats it would shut against the feed side considering pascals fluid dynamics while the case is pressurized...not the vacuum idea like i originally thought but it still would shut under the same operation since the tube goes to the bottom of the pan.

This is where removing the tube might be key...since fluid level might be below the drain level while in operation and not be exposed to the hydraulic displacement. I say might because we dont know whats going on while the car is in motion exposed to G force and the case isnt completely pressurized/oil pump doing its thing but I think it will be clear to drain.

So I think you and RabbitZL1 are on the best track.