1933 manifold wiper fitting problem

[RobbHelf]

Gentlemen,
I'm in need of direction on how to secure the brass tube that fits into the vacuum compression fitting for my wiper motor on my '33 manifold. I purchased the correct fitting from MACs that comes with the brass tube. The tube slides in fine and the fitting tightens up into the manifold nicely but when I can't tighten any further and I would assume I'm finished the tube slides out with little trouble. What's the trick to keep this air "tight"?

Thank you guys