3406b fuel line leak at head?

Check it and see it's a flare fitting. If its tight or missing some of the hold downs then thats a sign the flare is starting to crack

 

A lot of people way over tighten them and distort the flange on the line. I have ran into that many times after an inframe or injection pump replacement. Trucks leaves fine but comes back leaking. Retorque to spec and it won't move. Back it off and retorque. Many times they will get back to their deformed sweet spot and quit leaking. It may take tries over several days. Excessive tightening just makes it worse and you need a line.
The other method has worked for me many times. Just me experience and opinion.
Use a back up wrench any time you can.

 

Any time you are turning a tube nut put a wrench on its mating fitting when possible. Even though it may have a lock on it. If it turns at all a copper washer or o-ring under the fitting could break loose a create a new leak. Good idea to do the same when tightening.

 

If it has not been messed with recently it is probably cracked as Wore Out said and needs to be replaced right now. I would not go out of town with that.

 

You should be able to just pump the primer to pressure it up. Before snugging the fitting down pump the primer pump until fuel comes out, then snug it down. Depending which line it is, an offset 3/4 wrench may come in handy.