Brake line flaring is a process that adds a flare to the end of the brake line to ensure a leak-proof connection. You can think of a brake line flare as similar to the flare at the bottom of a pair of jeans. Flaring makes the tubing wider at the end. Because the tubing is wider at this point, the connection can slip in deeper and more easily. This is just one more way to guard against leaks.
Most of the brake lines you buy at an auto parts store will already be flared at the ends. However, those brake lines may not be the length you need, so you'll likely have to cut them and then add your own flare, at least on one end.
Brake line flaring tools are available online and at most parts stores where you can purchase other brake parts. In fact, most auto parts stores sell flaring kits that contain everything you'll need. The size of the flare is measured in degrees, and various car manufacturers use different flarings. For safety's sake, it's critical that you get the right size for your car.
Basically, this is the way to flare a brake line: Begin by using a tubing clamp to cut the line to the length you need. Then, before you make the flare, place the brake line fitting (which will eventually connect the line to other parts of the braking system) over the tube. Place the tubing in a clamp to hold it steady. Next, take the flaring tool and insert it into the end of the tube you want flared. The flaring tool looks a little bit like a pitchfork with three prongs. Two prongs attach to the clamp holding the brake line, and the third prong (the one in the middle) actually flares the line. It's mobile; you screw it in and out of the line, which widens it, making the flare.
A proper flare will be even and centered on the opening of the brake fitting. You'll want to take your time with this operation. The quality of the flare determines how well the fitting will connect the brake line to whatever brake part you're attaching it to.
Besides flares, brake lines also have bends -- lots and lots of bends. Keep reading to find out if you have what it takes to bend your own brake lines or if you should opt for the pre-bent variety.