Automotive Brake Line
Brake lines are made from double-walled steel tubing with one of two standard flares on each end. These are the double flare and the ISO flare. Never use copper tubing on automotive brake systems because it is unable to withstand the high pressure and vibrations steel tubing can. Some tubing is seamless, and all brake tubing must be double-walled. It is coated with tin or an alloy capable of resisting rust and corrosion. Brake line leaking can result from using the wrong type of brake line on a braking system.
Brake tubing is rigid, so brake hoses are used between the brake units and the fastened brake line. Never re-use the same brass washer when replacing a worn brake hose. Always replace these washers with the new ones provided. Armored steel tubing is used in places where road debris or heat can damage brake lines. This armor consists of a steel coil wrapped around the tubing.
Each end of the line is flared to either a double (not single) or an ISO flare. It's easiest to use a prefabricated line from the manufacturer that comes with the prefabricated bends and proper flaring and flare nuts for that model vehicle. If it's not available or if a line's fitting and flare needs to be repaired, tools are available to do so. There's a special tool required for each of the two different flares. The double flare flares out and then back in on itself. Notice the ISO International Standards Organization looks more like a bubble. A brake line nut or flare nut is used to fasten these lines to their fittings.