I have low air pressure on a 7.5 ton man lorry?

I Dont know if its faulty compressor, i dont have any test gear.The symtoms are when the air tank is full if you press the brake pedal 4 or 5 times whilst the lorry is ticking over the air decreases rapidly and the stop light comes on. The lorry is about 10 years old, the only thing i noticed is that i didnt hear the air pressure valve blow off when the tank is full