Easiest way, replace the fill valve, they are cheap.
http://www.evike.com/products/27269/ these generally will fit most gas guns with a few exceptions.
Next step, try taking it out and lubing the o-ring in the fill valve. A flat head screw driver and care is all you need to remove the valve.
Next step, try replacing the o-ring on the valve with one from a hardware store (they are hard to find because they are small).
Next step, disassemble the entire valve, yes it comes apart, there is a little brass end, or silver end, that will have a spring and the valve in it, might have dirt in there.
Next step, none of those work, refer to step one.