so is it actually the fill port that is leaking and not just the fill probe? I would say there is an o ring that needs replaced
Most likely the check valve o-ring is worn & needs replacing.Remove the reservoir from the gun & degas it. Place the reservoir securely in a vice & put a screwdriver through the fill port & turn counter clockwise. It might be a tad stiff'difficult if this is the first time it's being opened. The entire fill port would then just unscrew out of the tube.On the underside of the fill port there is a spring seat with 6 holes, unscrew it carefully & there is a spring below it that keeps the check valve closed, be careful not to lose that spring. After the seat & spring are removed, the brass check valve should just slide right out. Now that you have the check valve in hand it's time to change the defective o-ring. & clean out with a moist cloth the area where the check valve came out of. Try to avoid damaging the brass in any way as you attempt to remove the rubber o-ring around it.After the o-ring is changed, just reverse the steps taken & you are done, & once you use the correct size o-ring it won't leak anymore.