Catch can is sealed with an O ring. If I had a vacuum leak I'd be throwing codes. I'm not. The line essentially runs from the valve cover, through the catch can and into the intake manifold. When I first tested before the mod I got 1/2" hg vacuum. The system is definitely working as I get oil in the catch can. If I tested vacuum by porting directly into the intake manifold I would expect 17" or more.