The following method may sound a bit odd, but it works:

1. First swipe your thumb & index finger along the length of your nose. (This will pick up just the right amount of oil off your skin.)
2. Rub your thumb & finger on the brake valve fitting.
3. Using a minature forcep or similar small blunt tip tool, insert the end into the tubing, expand the tip, stretching the tubing slightly.
4. Remove the tool and quickly force the tubing fully onto the fitting.
5. Safety wire the tubing.

This works so well it is now virtually impossible to remove the airline. To remove without scratching the brass nipple remove the safety wire, then "cut" through the air line along the valve fitting with the small tip of a hot soldering iron.