You have consider how it was designed to seal. The original setup had a fitting with pipe thread on one end and a flare o the other. The pipe thread was designed to seal against pressure. Ordinary threads are not, they only provide a clamping force.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!