on my IBS install, I took the vent line from the infloor tank and cut it about 6" after coming up thru the floor infront of the driver, add a Tee fitting, one side of the T goes to the IBS lower port, the upper port goes to the vent thruhull on the side of the boat, this has a wakemakers inline check valve. On the other side of the T I ran a line up higher than the vent thruhull and added a spring check valve from Ace hardware or from Home depot-had it a few years and just don't remember where it came from. When the ibs is full it will flow out the vent thruhull. when empting the ibs will dang near drain 99%, then when the vacuum pressure hits a certain point the spring check valve will open and let the infloor tank drain about 90-95%, then when im on the ramp pulling the boat out I have my wife hit the empty to finish the job on all 3 bags as there is allways some water left. anyway this is how mine is and it seams to work pretty good-all automated, no valves to mess with. hope this helps, if any questions feel free to ask
I am glad to see my spring check valve idea worked. I will be installing it in the near future.