The return line(s) go back thru the fuel selector valve. When the valve is in one position it feeds and returns the fuel from and back to one tank. When it is in the other position it does the same for the other tank. I would only by pass the return if there was no other way. The pump draws more fuel than the engine can use and sends the excess back. Bypassing the return line would probably result in to much fuel going to the engine. Even if one of the fuel lines going into the fuel selector valve is leaking air the truck is still drivable. It’s not easy, however all one has to do is keep the rpm’s up. Those lines can draw air it just makes the truck a “you know what” to drive.
Without looking at it I should think that the fuel lines just needs to be primed to get the truck started. After that it will still idle a little ragged because of air in the lines, and the only way to get that out is to drive it. And if it won’t idle steady after you get it home it probably has a fuel line(s) leaking air at the fuel selector valve. Even if it doesn’t, do yourself a favor and replace them when you can, you won’t regret it.
