Chasing the leak can be a pain.
Have you tried locating the leak using some foam?
It doesn't have to be high end stuff. You'll get the same results using heavily foaming Fairy dishwashing fluid or similar. Just fill the rifle up and apply foam anywhere, preferably around possible leak points like the fill area and barrel tip. It will be of great benefit to try and plug them holes a little, like inserting the fill probe and even sticking an ear plug or something equally harmless in the barrel because the foam doesn't usually work on large openings. If and when the leak presents itself you'll know...
These are all simple tools. I don't know if You have gone this route already.
Maybe someone will take something from this and that is a good thing.
This type of problem solving will go a long way. In the distant future you'll know right away which O-ring to change and You can do it on the field. You need 5 minutes, hex keys and a rag
All the best