yep.. fire fighter has it right.
Put the fill line in the hard tanks, the vent line from the hard tank to the bottom fill of the bag. The vent out of the boat should then be connected to the top fitting on the bag.
The pressure from all the water in the bag will push water out of the drain pump and will take forever to fill. I rerouted the drain line up the outside wall of the locker (so I can just open the locker and turn a valve) and installed ball valves. I close them to fill the lockers and open them when I am done - water will stay in the bags when the pumps are off.
I used something like this... I can't remember if its 3/4.
http://www.homedepot.com/h_d1/N-5yc1v/R ... reId=10051Fittings to connect the ball valve to the existing tubing:
http://www.homedepot.com/Plumbing-Pipes ... reId=10051I ran it this way about to years ago and its worked good since. Its nice to be able to fill just the hard tanks for my wife and her friends to skate at low speeds and they don't get tripped up on the lip from running just the wedge.
Johnnydefacto installed electric valves to replace the manual ball valves.