I am using waterbed tubes and a twin waterbed mattress for my sacs, so they only have one valve on them. This means that I use the same line to fill and drain. I have a T fitting on it that goes to the fill and drain pumps to seperate the systems. In my setup, I could run a vent line off the main line to vent when the bags are full, but it would vent the entire time it is filling, therefore I need a pressure relief valve to allow the bags to fill first before the vent line opens up. I know it may be hard to follow, and I don't have the drawing of my system right now. I wil do another drawing and post it for you guys to see what i am talking about.