Steel fuel lines can be a prick.
Make it out of 3/8th and you shouldn't have problems.
I have made fuel lines out of 6mm rod and then used that as a template to bend the steel tube.
You can cut the 6mm and weld it together to get the shape you want.
A little pipe bender is the best to bend it with, they usually cost around $50.
To crox the end hust give it a little nudge with a flairing tool, like you were going to do a double flair but stop as soon as the tube starts to compress.