when I came out in the morning and one corner was down, that gave me a big clue something was perished and leaking air over the driveway. it came up with the others so pump and valve block were working. the tests for these are on the forum.
the car gives you warning, a sagging corner tells you its leaking. the airsprings have been on the car the same length of time, so if one is perished to leaking, the other three are not far behind. change them together.
the usual case is.. one airbag leaks it gets left for weeks.. the pump is feeding a leak, the airbag get replaced. the next airbag goes.. same thing.. by the time all 4 have been replaced the pump is now buggered from feeding leeks, then the system gets called unreliable.
this isn't rocket surgery.
rubber will perish at the same rate for all airbags, if one goes the others will soon too.. and if the rubber there is perished you'll be assured the O-rings in the valve block will be perishing to hard as well. so change them. its a £30 kit and takes about an hour with a cup of tea. same with the compressor piston sleeve.
maintaining is thinking about what has been done and when last. you don't change your gearbox oil when the transmission dies, or the engine oil when seizes ? if the EAS is dropping a corner its leaking air. something has perished. its all about the same age so change it *before* it fails.. that's maintenance. changing 'when' it fails is repair.