Yes, the pump has inlet and outlet check valves. Use a pressure gauge on the carb inlet line to check the outlet valve. Run the engine for a few seconds with the carb inlet line disconnected, and the pressured gauge attached. The pump should hold pressure for a short time after the engine stops. Then go to the inlet side of the pump with a vacuum gauge, and run the engine again. The inlet check valve should hold vacuum for a short time after stopping the engine. If you go all the way to the fuel tank, and run a vacuum check where the line leaves the tank, you can check for restrictions and/or air leaks in the fuel line. At the tank, a somewhat lower, but steady vacuum reading should be present, also with a short hold after stopping the engine. I'm betting you've got an internal leak in the carburetor, resulting in an empty float bowl.
BTW, a little air bubble in the filter, or the glass bowl, isn't a problem. Most of them do that, even while running, in some cases.
Jerry
