charlie, if there are enough "floaties" in the gas tank (pieces of trash, small rust particles, but they don't float, they settle to the bottom where the pickup tube is, etc), you almost have the milkshake straw affect when the fuel pump is sucking gasoline with junk from the gas tank. All of the trash goes directly the suction, or outlet. It's almost like getting a Strawberry Limeade at the Sonic Drive-in, and then sucking up the junk at the bottom. You can blow through the straw and have plenty of passage, but when you reverse direction, all of the junk gets back in the pickup tube...
the last time this happened to me, I had to remove the gas tank, take it to a radiator shop, have it cooked, sealed, and a new pick up tube welded in. Worked like a charm.
Daniel JessupLancaster, California
aka "The Hot Rod Reverend" 
check out the 1955 Ford Fairlane build at www.hotrodreverend.com