the fuel pump is comprised of 2 check valves, with a diaphram in between, and the fuel evaporates it doesn't drain back, as the needle valve (inlet to the carb) comes in near the top, unless you put a dip tube of some sort in the carburetor it can't siphon it back out. If your filter is draining, there is either some little pieces of stuff in the fuel pump or the valves are faulty. Mine does the same thing if it sits for more than a week or so in the summer, but once I took out the little pieces of gunk in the pump the filter stays full (this has happened a couple of times) By the way is your garage heated? That might explain how it evaporates so quickly.
