Ballast in rear ski locker on DD boat

I'd like to put a fat sac in my rear locker. The locker is above the gas tank. How can I plumb it so as to prevent siphoning through the vent line and drain lines? The ports will be slightly below the top of the sac. I plan on using a Tsunami pump in the bilge to pump water into the bag, and another Tsunami pump attached to the bag for draining.

Thanks

Trash

Run the emty and vent lines to some point in the boat that is higher than the top of the sac and then run it back down to the thru hull from there.

I don't think that will work. If I fill my bag so that it's full (and water is coming out the vent line), then it will still siphon from the bag. There is no air in the line to break the siphon.

Trash

How about check valves.