I'm sure the book would say to replace the entire return line...$$$

Are you sure there's a disconnect down under the MC? I'd think it's just a nylon-to-steel "overlap joint" transition.

Maybe spray half a can of carb cleaner into the line and try to blow some air through it? Or maybe get a tiny little funnel, pour half a bottle of injector cleaner into the line, let sit overnight, then blow it out?