I'll give it a shot.
Ok, it's kinda hard to troubleshoot without being there, but my guess would be that the valve has not been oiled recently. Try that first. Just put in a few drops and then cycle the gun to get it into the rings inside the switch valve (3 way).
If this doesn't do it then you may need to adjust your timing rod length. I'd guess lengthen it to help it fully close, but it could go either way if it's leaking out the front too. play with it, but remember this will possibly require you to retime your gun once it's adjusted.
Third diagnosis, is that you could have one or more bad O rings inside the 3 way. Pull out the valve stem and look for ANY irregularities (cuts, scratches, nicks, etc.). If you find anything you probably need a new ring. Usually this problem will also mean your gun does not fire correctly or complete a full cycle. The trigger may not even return forward!
If it's none of these things check back in here and let us know how it went, or get someone who knows. The best way to learn is to do it yourself but it can be a long and painful journey.
Oh, and make sure you have the hoses on in reverse order from the stock 3 way. Bomb valves are a little different but that's what makes 'em special.
