I would spray the valve and fitting with soapy water and look for bubbles. It would probably be best if the the engine was running and the system in "Travel" mode. This would apply air to the ride height valves, air line and air bag.
In "Leveling" mode does that air bay loose air if you turn off the auto leveling?
2000 Country Coach Magna
CAT 385HP Allison 6sp
1998 Saturn SW2