I don’t think anything is wrong with the gauge port assembly, it is doing exactly what we want when someone is pumping water into the gun. We have seen a couple of these units rust and it is more to do with the hand pump and how and where it is used.
The best solution is to steer this customer to a Compressed air tank set up. If they had this issue in just three months of use, they must be using it multiple times per day, every day.
One thing to recommend is the user take a break after 50 pumps to let the pump cool down. Also operate the pump slow and steady when pumping. The hotter it gets, the more moisture it will generate. When bleeding the hand pump, they should open the valve screw rapidly, so that the moisture will be blown out of the base.
Any pump on the market will pull moisture from the air if it is hot and humid. One way to relieve this is to use the hand pump in an air conditioned area.
This being a .25 Caliber gun isn’t helping things, as you get fewer shots per fill.
I still think the hand pump is one of the better solution for filling a gun. We have sold a lot of pumps and the majority of the users have not had this issue. In some cases like this a Scuba or HPA tank may be better suited as the primary means of filling the gun, and keep the hand pump as a back-up.