When I installed the new line, I was unaware that there was supposed to be a rubber line between the petcock and the steel line. I just hooked up the new line the way the old one was. I haven't done anything to change it.
Interesting. Usually there is a short rubber section between the frame-mounted hardline and the petcock for cab movement, then another between the frame-mounted hardline and the fuel pump for engine movement.
I am kind of surprised you have not had fatigue issues with those both solid.