The mode really has nothing to do with the valve. It mearly provides a stop for the trigger so that it cannot hit the trigger rod while in E mode. In M mode the trigger stop is removed and the trigger can move the entire distance to push the trigger rod and allow a mechanical pull. If putting the emag in M mode causes a leak, I would suspect that it is level 10 related due to a tiny bit of force on the trigger rod. Remove the powertube shims if there are any present.
Except for the Automag in front, its usually the man behind the equipment that counts.