One of the big failure points of older air suspension systems is the piston ring on the compressor getting extremely worn and not able to seal in the cylinder bore which prevents the pump from generating the pressures needed for the system to operate.
Mild pitting and scarring of the cylinder bore can be remedied with a light buffing using fine sandpaper.
After changing my compressor piston ring my system was restored to full operating status. Below you can see how nasty the old piston ring had become.
Mike