[Opea]

it could be one of 2 things yeah, the big orings that seal the bottom of the mag, or the valve. My suggestion would be to take it apart (to fix it your gonna have to anyways) take the big orings off, 1 for sure, might be 2 of them, soak them in some silicone oil. Then try that cdn stalker suggested, work the valve with some oil, maybe even put a bit of oil on it and leave it over night. If you take your valve out be careful not to damage it, its not very strong metal.

I have had mags that leaked in the past, jhust need to keep the orings oiled up and they will seal. If they dry out they are useless. Make sure you add a few drops of silicone oil to your propane every couple of mags worth of shots.