Flaring tubing for brake lines can be frustrating. While a simple concept, it is critical that it is performed exactingly so as not to compromise the integrity of the tubing, which could create a failure. Failure in the brake system could have catastrophic effects, and so it is critical that every element of work on a vehicle's brake system is exact and perfect. Double flaring is a technique by which a flare is created at the end of the tubing, then the flare is turned back on it's self, creating a double walled lip at the end of the flare. A double flare technique is SAE recommended for brakes and air conditioning systems in vehicles. Here you will find some tips to creating a double flare on the end of brake tubing, and some precautions to take to enable an ideal outcome, a perfect double flared brake line. The common thread in flaring brake lines is getting the initial flare perfect, doubling the lip back over is the "easy" part.
Using the right tools seems to be the common thread that ultimately will give the best possible outcome of obtaining a good flare on the tubing. There are several different types of flaring tools. Older models have a cast in which the tube is placed and a mechanism that descends into the tube to gently open up the edge to either a 45 or 37 degree flare. There are several variations on this type of tool, but all work the same. The other is a hydraulic action flaring tool. The consistency of the pressure and the smoothness of the operation of the hydraulic tool seems to give more consistent accuracy. Additionally, the right amount of force is needed to ensure that the tubing is not thinned at the flare, and is a common occurrence. A hydraulic tool also helps with this problem. Hydraulic tools however do cost quite a bit more.
Cutting the Tube
A perfectly perpendicular cut of the tubing is critical when flaring the tubing. If the tube is not cut at a right angle, the flare will not be even, giving an irregular lip to the flare. This will compromise the fit of the flare with the other fittings, and simply makes the fit not work. There are several good tools on the market to ensure that the tubing is cut precisely. There are stocks that can be used to make the cuts with a hacksaw for other metals, but generally brake line tubing is soft and pliant and will not hold up under a hacksaw.
Once the tubing is cut, any burrs will need to be removed by filing them down. Any burrs on the cut edge of the tubing will interfere with the flaring process, and if the initial flare is faulty, the double flare will be as well. There are a variety of tools, including files, on the market to remove burrs and smooth the edge. Care should be taken when removing burrs not to thin the metal of the tubing at all as this will compromise the integrity of the tubing, and can cause it to split.