Can;t get gas up to carb

well I took a second look at the sending unit. I have the return line coming into the top of the tank and the gas line going into the sending unit, and then there is another line on the sending unit that has a cap on it. Is this for some kind of return line as well.? It has a plug on it right now that was leaking, probaly creating a vacumn leak. Is this the wrong sending unit for this application? I see them online with and without this line on the sending unit. My 72 scamp has just the one on the sending unit and vent on the top of the tank as well. I am working on a 72 Duster.