Back in 2013, I built my return assembly for the 400g version 2.0, and when I did so I used a check valve installed backwards, which I refer to as the Reversed Check Valve. When the return pump flows, water pushes the valve shut, but as soon as the pump has been powered down (intentionally or unintentionally), the backsiphon of water sucks the check valve open.  When this happens I hear a brief sucking sounds letting me know the siphon has broken.