The fuel pump has two one-way valves that would ideally prevent such a thing. Absent a defective fuel pump it's usually the simple fact that the gas evaporates out of the float bowl. If you don't have the symptoms of a fuel pump going bad (running out of gas under load and/or when hot) it's likely the latter and just evaporating. This is a long standing issue with old cars that sit for days/weeks at a time. Many folks put an electric fuel pump in line between the tank and mechanical pump and wire it to a momentary switch to prime the carb. I did that on my 53 but have not yet done it on my 38. There was a recent article in the G&D about doing just that.