Daf 45/150 air build up?

Hi guys
I have a ex MOD 52 plate Daf 45/150 that slowly builds up air on both tanks up to 5 bar, but wont go any higher , so needless to say brakes wont release and buzzer is on, there is no obvious air escaping from valves, brake chambers, etc, that i can hear or feel. and no air at handbrake ,Im presuming that would be the "second stage" build up, the Governor appears to be above the exhaust and is non adjustable its wired to ecu, i think.
Im a retired guy looking at this for a friend ,its a long time since i was a HGV fitter. any help much appreciated.thanks
Fred