By placing a tiny bit of pressure on the sear, you are pushing the bolt back just a bit, just because of the angle of the sear. That is allowing the level 10 vent hole to go behind the sealing edge of the carrier oring, thus sealing the leak. Too many shims would cause that issue, which is why Tunaman mentioned the shims. It would have made perfect sense, and it would have been my guess as well.
Is the rail bushing in place?
If you push on the back of the valve, does the leak go away?
Except for the Automag in front, its usually the man behind the equipment that counts.