Two problems:

One, air suspensions always leak eventually.

Two, I swear nearly nobody in the industry tests anything, it seems the normal thing to do is replace the obvious thing, be it the pump or a bag, and try again if they were wrong instead of diagnosis.

For example - and this is just my experience with four F-150s across two generations of truck and three dealerships - Ford, of all things, has been running these stupid ass hold-off vacuum 4wd actuators forever. They all eventually start leaking, and your front axle engages so the truck grinds and vibrates. They all do it, over, and over, and over again. I mean they start leaking at like 50k from somewhere.

Thus far 90% of the time it's been one of the check valves or a split line. Thus far 100% of the time the dealership will replace both actuators first and send it. Under warranty no less.