I tell this to everyone who has a leak.
Plumbers silicone grease.
This stuff is heaven in a bottle. It'll save any leak your pistol has.
Simply unscrew the charging valve or gas release valve (Which ever leaks, although I would reccomend doing this to both) and then simply roll them in the silicone grease.
This will grease up you're O-rings and prevent future leaks.
The stuff is mabye $4 at Home depot, but make sure it is PURE silicone. Check the ingredients. Make sure there is no petrol products in it.
Most likely, any bottle you pick will be pure because it needs to be safe to work on PVC pipe.
I've been using this stuff for years, and it truly is amazing.
Pick some up, it's worth it. It has other uses too. it works good for lubing AEG gears, pistons, GBB slides, and other parts that experience friction.
It's the best $4 you'll ever spend towards airsoft.
Good luck,
-B00M!