There are only a few things that can cause a solenoid leak, at the proper pressure range (70-220psi), that I have found. The most common cause is of course the main gasket. The trouble is, though, they all seem to feel like they are coming from the base of the solenoid so you can't diagnose the exact cause any other way that swapping things around. The first thing I always start with is replacing the main gasket. Simply put a light film of grease on both sides and gently tighten down the solenoid by screwing one side down until you feel a little tension, then tighten the other side down all the way (not TOO tight! Those threads are TINY) then tightening down the other side as well.
If it still leaks try swapping around the middle o-ring on the ram assembly with the one closest to the hammer. If this fixes the solenoid leak but creates one coming down the barrel then you know that o-ring needs to be replaced. If not, it's time to crack the solenoid open.
The first thing I look for is a backwards butterfly gasket (rounded edge touches metal). If both are installed properly then I look to see of the spring is properly seated on the spool. Then the gaskets on the spool its self. Be sure to look for any hairs or debris while in there, they will DEFIANTLY cause a malfunction. If all looks good I check the o-ring between the coil and middle section of the solenoid; as well as the spring in there. If it all still looks good, start replacing things. I'd start with the butterfly gasket on the side between the end cap and manifold. After that would be the other butterfly gasket, followed by the spool, then o-ring between coil and center piece of the solenoid. After that, just replace the whole solenoid. Here