Easy way to check-put the vehicle at its highest ride setting (raise it), spray all the air bags with a dish soap and water solution, watch for bubbles. The rubber actually dry rots just like a 8-10 year old tire, they crack and leak right in the fold of the rubber bladder. If you don't find a leak in the rubber bags check the valve block with said solution, but 99% of the time the bags are leaking.