First thing you need to do is diagnose where the problem lies.
If you hold the trigger in all the way, does the leak stop or does it sputter? If holding the trigger stops the leak, then your issue is the powertube area. Most likely, your powertube oring or perhaps your spacer is too long. The spacer issue could rear its ugly head if your sear is worn a bit and then got suddenly very slippery with paintball gunk.
If the leak doesn't stop when you hold the trigger, and just sputters, then you have a leak in your on-off area. That would most likely be small inner white teflon oring on your on-off top.
Except for the Automag in front, its usually the man behind the equipment that counts.