Benjamin discovery fill valve leaking?

There is a check valve in the Forster fitting. Take it out and make sure there is no dirt or something else holding the valve from seating. Make sure there is no air in the gun first.

Jonah is correct in the foster fitting but I would advise to be sure you have the correct sealing tape to seal the threads when you reassemble it. The sealant on the threads now will likely fall apart and disintegrate when you unscrew the tip and are unlikely to seal it again.
Regular white teflon tape wont do the job, I used a much thicker black silicon tape that is very stretchy. It is sometimes refereed to as mocap tape.

Now that you have the endcap off I would also take a close look down near the pressure gauge to look for any rust/corrosion buildup from moisture collected inside the airtube if you are using a hand pump. I had enough corrosion that it ate right around the seals and I need to replace the airtube and other internals....