Brakes hissing?

I took Ford Iveco horse lorry out on the weekend to a competition and when i arrived at the comp, switched the engine off, the air system was hissing sounded like a small set of bag pipes coming from the air compressor system. This hissing continued for the entire time i was there approximately 5-6 hours, and again when i returned home, what could be causing this? there was no lights on the dash to suggest a problem however the pressure was close to 9 Bar on the gauge, 12 is the maximum on the gauge is it perhaps struggling to release the air? PRV fault?

i'm not sure...