air gauge is fed from air system(depending on variant),to gauge.lift front grill,left side distribution rail to behind passenger bulk head,lead through to driver console and direct(to gauge(early level).otherwise circuit protection valve area.(again depending on level).have u done an isolated air pressure test?continuity test relative harnesses.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lorries/trucks.