I had a similar discovery as Mapleack. I have an 8' star lift with a vac. gauge at the bottom and a valve to drain the lower mainline. I run ~25" in the woods. I first tried it out without venting it, and I also found the vac. levels much lower...say 18" because the lift would plug up solid. I kept it closed for a period of time, and the vac. level was always much lower than the rest of the woods. I barely cracked the ball valve open and the vac. level immediatly jumps to 21-23" once all the sap clears out, and it stays there. Based on this, the valve stays open just a touch.

I have valves at both ends of every mainline, so I can clean the lines out after the season. One thing I will be trying is leaving the valve closed at the bottom of the lift, but opening the valve at the far end of the mainline. I've been told this will maintain higher vac. levels on the mainline, while still giving you the air leak. We'll see.