My '80 has 3 lines running from the fuel tank. 2 of those go to the fuel pump & the smallest one(1/4" I think) goes to the vapor cannister.
 
The 3/8" line is the fuel feed from the tank to the pump. Is that smaller 5/16" line running from the pump back to the tank a fuel return line or some kind of vent?
 
 

So there's an internal pressure bypass inside that fuel pump? What was the reasoning? Did it prevent vapor lock or solve a flooding problem? I'm not running the stock '80 carb anyhow, it's getting replaced by a new Holley 350.
 
I wonder if I should change to an earlier pump that simply supplies fuel & eliminate the return. I was planning to move the vapor cannister to the rear & shorten the line up anyhow so I can remove the return while I'm at it.