This is only a possibility. You mention changing the air processor. Was this because you could hear a very slow leak probably coming from the middle of the valve between itself and the air drier? If there is a leak elsewhere in circuit 1 (rear service brake) then the air processor will bleed back circuit 3 ( hand brake). Possibly you still have a leak in the rear service brake circuit. DAFs are prone to corroded air tanks particularly with pinholes around the ports and more cheaply weeping drain valves.