A-904 TC fill quandry

On a A-904, like the 727, the torque converter doesn't fill in park,
only in neutral, which is a really good safety feature, if you stop
and think about it. I notice after sitting a couple of days, if the
car is started in park and moved directly to either rev. or for.,
there's an initial "bump," not too hard, but firm. When it is first
shifted to neutral to fill the converter, initial engagement is
smooth. Once the car's been warmed up and driven, further engagements
are perfect. To proove that this may be part of the bumpy engagement
when cold, I put the cooler supply line into a pan and listened as it
was filled....sounds like a small amount of air is burping out of the
converter when first shifted from park to neutral.

Is this common, or is there a leaky converter check valve in play
here? Any "easy" fix for same, or just the nature of the Torqueflite?