Why is fuel pump pumping air?

1977 Ford LWB F250 351-400 This truck has two fuel pumps. An electric in the tank and a mechanical on the engine. As far as I know this is stock setup. Truck has been sitting for about a year without being run. Empty gas tank. I put in 4 gallons of gas, cleaned plugs, new battery, etc.. Running a little rough, but running. I have a clear plastic inline fuel filter just before the carburetor. With the engine running, I can see air bubbles going thru the filter. Filter never gets full of gas. These bubbles are as large as the fuel line. Maybe 3 or 4 per second. I am surprised the engine runs at all. But I'm guessing that enough gas is getting to the bowl so that the air bubbles are not a problem. Question: where is the air coming from? Thanks, Chief