Number one reason mags leak is because they were stored without ANY gas in the mag at all. I've seen guys end of the game day when packing up their gear empty the gas out of their GBB mags, I'd ask why, they said they didn't want their mag damaged by keeping gas in them. They were shocked to find out the damage comes from no gas pressure during storage at all. I've always kept gas in mine, even if the mags sit for months unused, they never leak (I've got 8 KSC G19 mags and two KSC Mk23 mags). And the most common place that the leakage would come from is the main gasket at the bottom of the mag, it'll shrink a bit because of no gas pressure, and cause the leaking.
I've fixed a good dozen GBB mags over teh past couple years, all same problem, and I found the best way is to use the blue gasket maker gel found in the automotive department at Canadian Tire. Remove the bottom of the mag (often found by punching out two pins), smear a good amount of the gasket gel around the rectangular rubber gasket, reassemble and let sit a full 24hrs before putting any gas in. Then fill and test out. If you put gas in before the 24hrs is up, it won't cure properly, and if you test fire the gun during this period, you'll get vapourized gel all over the insides of your gun........... totally not fun to clean out!!! (How do you think I learned about the 24hr waiting period?