I find that the Saf-Air CAV-110s are a pain. They like to leak after
I have replaced the O-ring with one made of Viton, which has helped, but
put off using them for fear that one day (especially when on a trip)
will prove impossible to reseat.
When, in a previous life I flew almost every day, each time I checked
the fuel, I always pulled downwards on the fitting giving it a slight
turn as I did so. Doing this helps it seal and is the reason I have
left mine proud of the fuselage.