Whenever my SVI mags leak, I just disassemble them, get the rubber parts all silliconed up (except for the port on the top, the part that actually makes contact with the gun's nozzle) and let them soak for 10 minutes or so. Reassemble and they should be fine.
I find my mags only leak when I've left them un-gassed for extended periods of time, so just keep them under a little bit of pressure.
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