I was having a prob that sounds similar to yours. If you take the whole gauge off the top of the pump there is a little plastic insert that goes into the top of the narrow tube adjacent to large tube. You can see it when you get the gauge off. If that insert is worn or not in tight, me thinks you get a leak.
Topeak support sent me a new one, but it took a while for us to figure out which thing was contributing to the leak. They actually sent a whole new gauge...which was not the problem.
