2.0      PROBLEM: Air is building up in one gauge only
2.1	System protective valve	Leak through element 21 or 22. Value not functioning.	Overhaul assembly.*
2.2	Dual brake valve Relay valve - if fitted	Leak through exhaust or unused ports not plugged.	Overhaul assembly*. Plug unused ports left open.
2.3	Air gauge	Defective air gauge	Replace defective air gauge.