Hi @DaleKramer: I did not know there are any edit limits: hope it is solved for you…
From observation of small BMB (velocity vectors or particle traces) I see that those BC defined as Zero Gradient are ‘leaking’, so air or water will travel the BC in both directions. This is pressure-wise as kind of surface tension prevents complete fluid evacuation. In any case, most of fluid will still be evacuated by Pressure outlet.
As we do not have programme insight into Zero Gradient BC, this is just the ‘intuition’. May be somebody from inner circle will help to understand the real nature of Zero Gradient BC.