I assume it is a flared steel line, and if so, you should be able to add a piece of the same kind of flared steel line to extend your existing one that is too short. You can connect the two pieces with a brass coupling (flared female both ends) that is available from Ace Hardware or other brass fitting suppliers. NAPA sells the flared steel lines in various lengths from about a foot up to four feet. Check the diameter of your existing line before you purchase any additional.
If you or a friend have a tubing cutter and a double flaring tool, you can cut the new piece to the exact length and re-flare according to the instructions on the flaring kit. Make sure it is a double flare. I built my entire brake system like this on my street rod in 1975, and have never had a problem.
Do not use copper, aluminum or rubber to make up the 6 inches.