found something out this weekend after finally getting to use my new ballast system, I added one way valves to all three vents. rear bag sucked down like a vacum bag--awesome results. IBS bag did the same.

this is where it gets interesting.

I got home and got the boat wiped down and stowed into the garage, all was nice and quite and I swear I could hear the boat burbing, locate source of burbs to the IBS bag that is piggybacked off the center tank, undid the quick disconects from the IBS and heard a long vacum of air entering the center tank, I then hit the center empty button, about 5 minuts later and the center tank was empty and the garage floor was getting a nice lake water rinse down. May have to remove the oneway vent or get creative allowing air to enter the tank as Im thinking a tank needs to vent to empty. Think I may add the directional valve and once the IBS is flat switch it so the tank can vent freely.