A poorly seated, improperly installed, warped, or dry pump membrane most certainly will permit fuel to flow back into the tank. If the metering needle was too high I'm surprised the carb wasn't dripping out the venturi onto the ground. A bad pump can lead you to think it's working because fuel can move through the line, but in an amount greatly reduced from what it should be. If the pump is working like it should fuel will remain in the line.

Since the carb had been "rebuilt", but the metering needle set to an incorrect height, who's to say something else wasn't also done wrong during the rebuilding? I can think of about 6 places in the carb where something can be improperly installed, screwing every thing up.