Check valve question?

I'm plumbing in 2 Johnson reversibles. 750 in each rear locker. A 370 under passenger side seats( our crew only surfs on the reg side) with a selectable diverter valve. If i run a check valve for the vents wont i need 2? One for each bag so that they dont pull from each other when draining, then out the single portion of the Y out a single thruhull? Im also going to have another selectable diverter on the other side running to a hose that i can pull into the boat for an additional bag for anywhere else in the boat. Hope my question is clear.

I don't believe you will need the two check valves. Just a single check valve after the two vents become one before they discharge out the side of the boat. When draining the other bag will be a closed loop and the water should flow as intended.

Depends - if you want no chance of water from the vent filling into the other bag, then you want a check valve before each side of the Y. If that isn't an issue, a single after the Y will suffice.

Since you are sharing vents, make sure it is at least a 1" vent, as a lot of water could be coming out at once. As for diverter valves, the Sea-lect valves work fine but the Fly High valves are really nice. I have one of each and plan on swapping my other Sea-lect for the Fly High valve. I also use dedicated thru-hulls for each bag vent as it isn't much expensive. Stainless Steel thru-hull was $13; Y-valve is $8. A little more labor but a cleaner install.

I wasnt worried so much about filling as i was draining.