underbed fuel sending unit

[blackula72]

I have a 72 swb fleetside small block with automatic. I'm trying to purchase a new under-bed fuel sending unit. the unit i took out has only one fuel pickup line. My search of replacement sending units have two lines, A pick up and a return line for that year.
Could someone out there explain the reason for a return line?
Since buying the truck a few weeks ago, the gas guage was one of the problems with the truck.