IBS plus Hard Tank. Possible solution.
When adding an IBS to a soft center bag vent, the installation is very simple. Install a check valve on the vent, and both bags will completely empty and suck flat.
When installing an IBS off of the hard tank vent, this set up will still suck the IBS flat but the center tank can not. So you end up with water left in the hard tank and your ballast pump may cavitate once the vacuum builds up in the hard tank.
I think someone on here used a spring check valve as a vacuum breaker to overcome this, but I can't remember who.
I was thinking earlier that if a company makes a plastic diaphragm check valve, surely someone makes a vacuum breaker as well. I found one here. http://www.plastomatic.com/vbl.html
I am going to try one added to a front sac teed off of the vent line. This should allow the IBS to be sucked flat and allow just enough air in to avoid a deep vacuum being drawn on the center tank.
If it works, I will talk to Wakemakers about this simple fix, and let you guys know.