It may not even be the valve that's leaking and the actual cylinder it controls… I would try and isolate which valve… stop the press and lower the air pressure to that block, then starting from the bottom of the block remove one air line at a time (the return side which in my case is the left side of each valve) you should not have any air coming out of the line, or the valve. I'm attaching an image of the valves and what they control again for my press.. Yours will probably be different.