The fitting advice is excellent. Proper sealing of the threads is imperative.
Every leak of the tubing fitting I have encountered helping friends etc has been improper prep of the tip of the tubing and making sure that are NO scratches etc on the end of the tube that is going into the fitting. An extremely smooth cut is important.. Just a bit of silicon grease makes the tubing and the "O" ring slide and seat easy and no rolling of the "O" ring..
I have rescued many folks with leaking Ice Maker Tubing attachments... SAME issue.
Think disc brake piston seals., if you have ever rebuilt one...
My AirLift bags, remote control, pump has been on 2 trucks and is 14 years old... NO leaks.
Jim