I don't know if the larger Mags (.40 and bigger) do this, but the smaller Mags I use in combat (.28/.25/.15) all have problems with air leaking around the o-rings on the high and low speed needle valves. It doesn't show up with the symptoms you metion (not shutting down), but as erratic running. Some fuel tubing fixes it right up. You can also replace the o-rings with larger, better quality o-rings, or do a couple of other things to improve the seal.

In general, if the engine runs fun, but won't shut down, the leak is pretty small, and probibly not in the needle areas like I mention, instead, I think Campy has it right. If it was me, though, I'd just tear down the engine and seal everything back up with RTV. After all, putting a bit of sealant on something that didn't need it won't hurt anything .