Think about the geometry for a moment. What exactly is pulling the tube out? I can think of two things:
1) the stem is too long so screwing in the crown is pushing in the stem, and it bottoms out in the movement, applying an outward force on the stem/crown/tube. Or...
2) the crown is too "wide" or the tube is too "short" so screwing the crown completely down has the crown hitting the case. IMO the crown should compress its o-ring and bottom out there, just shy of touching the case. In that configuration there is nothing applying an outward force on the tube.
Hope those ideas help?