I don't have any experience of the DRG valve, but from the look of it I assume it has an internal disc that rotates to progressively expose holes in the underlying seating plate, and allow steam to pass. The disc in these valves are generally not fixed directly to the operating shaft, but are held in contact with the seating plate by means of a spring. Incoming steam normally comes in on the rotating disc side of the set-up and keeps the disc firmly pressed against the seating plate. If the steam comes in the other way, it will still work, but will tend to lift the disc off the seating plate, and allow steam to escape round the disc, particularly when closed down. From the photo you posted, I wonder whether the valve is working in this fashion, as I would have expected to see the steam coming in on the same side as the disc operating mechanism. If so it's just neccessary to reverse the valve so steam is coming through the valve in the other direction.
As I say, I don't have direct experience of this valve, so please feel free to shoot me down, blow raspberries, etc!