The proper way to double flare brake tubing is to use a flaring tool. The tool is designed to flare the mouth of the pipe and create a seal that withstands the compression of the brake fluid without leaking.
Tools
You will need pipe cutters, an inner/outer reamer and a flaring tool kit. The kit contains three functional pieces; a clamp, with wing nuts or a clamp screw that tightens it onto the brake tubing, adapters that measures the exact length of brake tubing to make a safe and proper bevel and forms a bell in the tube mouth that prevents cracking, and a flaring tool. The flaring tool itself is a cone connected to a feed screw that tightens into the brake tubing by a metal bar at the top.
Tubing
Cut the brake tubing straight across with pipe cutters and remove the shavings from the end of the tube with the inner/outer reamer. Slip the tubing through the hole in the clamp that has the same diameter as your tube and measure the length against the wide step on the bottom of an adapter. Tighten the clamp onto the tube.
Adapter
Turn the adapter over and sit the stem of the adapter into the center of the brake tube. Slide the flaring tool onto the clamp and position it so that the cone tip rests inside the adapter. Tighten the cone down onto the adapter until it presses firmly against the clamp.
Final Steps
Release the pressure on the adapter and remove it from the brake tube. The mouth of the tube should now be bell-shaped that keeps the tube from cracking. Re-insert the cone into the brake tube and tighten it down into the tube as far as it will go to flare out the mouth of the tube.
