Ballast issues -- check valves?

I installed a Jabsco Ballast Puppy and Fly High Integrated bow sack over the winter. I used rubber-flapper check valves to push water out the side when draining.

The system worked fine the 3-4 times we first used it. When trying to fill the bag this weekend, the Jabsco was sucking nothing but air. I think my drain check valves aren't closing all the way when filling, causing air to get into the system while filling. There are no leaks in the hoses or fittings.

After searching on here, it sounds like "swing check valves" are the way to go... just need somebody to confirm that check valves are likely my issue. Or if there is something else I should look at?

Thanks for your help in advance!