Mr Monkey Nuts

Its a labour of love my friend, just think of the gasps of appreciation as you let rip in full recoil fulfilling, noise generating gas blow back mode!

And get some maint gas!

Tried that the other day but it still leaks from the filler valve

filler valve is potentially more difficult, you can unscrew them entirely with a small flathead screwdriver, give it a bit of a grease and put it back in. The only thing that could happen to it that'll lead to a new valve being required is a crack or the spring being snapped.