In fact there is a very small o-ring on the pin on the inner side. THis pin is pushed back by the big spring, which closes the valve on the one hand but also holds the small o-ring in places to seal the pin.
If this is not seal you can replace the small o-ring by a new one, or install another, a bit smaller o-ring just in fron of the original one to get a better seal. You can also install a STRONGER spring inside the chamber. More force on the o-ring will also stop it from leaking(in case the o-ring is ok and not ruined)
CLeaning the whole thing is also a good idea as there is dirt and other unnecessary things dumped over the years of using.
Try all this, be careful with using glue to not stuck the pin^^
THe fact that water is coming from the pin when the trigger is pulled can also mean that the "trigger-mechanism" is not straight anymore. My the metal stab that connects the trigger with the valve is courved a little or not on it's way, may it is out of it's direction ? The stab can be shaped with a little force, may this solve the problem too.
This is all you can do, I managed to get every pin-leaking solved and seal again, any of these tips wil help
Good luck!
A gunner has to do what his Soaker requires...