If you're trying to replace a new line with an old broken line it won't be same length as your other used rear line would have shrunk over time. Best to change lines in pairs or better yet, replace all the lines.
Broke one rear line and one front line, replaced both rear now have a spare. I replaced both front lines earlier but kept the old one, I re-used it and length is still same.
Has anyone had an instance of the bungee safety line breaking?