Would This Ballast System Work??

I was thinking about adding some extra sacks on top of the stock hard tanks on either side of the engine in the mighty Malibu Wakesetter VLX. I'd like to have them fill at the flip of a switch rather than doing the pump-in-the-water thing.

The simplest way I can think of doing this is to run the overflow hose from the current hard tanks directly to the bottom of each of the sacks. That way the sacks will fill up after the hard tanks are already full. Then I would run an overflow from the top of the sacks out the stock overflow holes on the side of the boat.

For drainage, I'd be counting on gravity to send the water back through the line from the sack to the hard tank as the hard tank is drained via the stock pump.

This sounds way too simple for it to actually work. What am I missing?