However, they don't do much when emptying the bag to keep air from flowing back into it from the vent. I still got a lot of air back into the bag -
This is the exact scenario I described earlier, that some experience with the Bosworth (again, sold by wake makers) one way check valve.

They won't stop passive drain, regardless of which check valve it is, because the flow direction is from the bag out the vent.
Absolutely correct, with a flapper style like the Bosworth valve. However, we have had great success stopping passive drain in the vent line. We have even had good results using them in the drain line on aerator system, so reduce passive draining from a pour to a trickle.

@ Shoebox,

You still got some season left? What size is your vent line? Ill send you a couple to try if you like. PM me your address.