Thanks, Mikes! I grew up tearing 3-speed sticks apart and never really studied automatics. I had wondered about the converter, but thought of it like a filled bottle standing up in a pan of water, staying full since no air could get in but it makes sense for the converter to drain back, I suppose back through the pump. It also explains why if I start the truck and immediately drop it into reverse (big 10 speed) there's a bit of a sluggish lag for a few seconds as it starts to back - the torque converter is operating with a partial fill, and as it fills it gets more "bite" from the fluid friction.

I'm hoping the pair of slightly larger O-rings seals the filler tube, and I may add 8 or 10 ounces of fluid just to check! Thanks for the input!