Brake lines carry brake fluid from the master cylinder to the brakes. They are basically the same on all brake systems.
For most of their length they are steel, and attached to the body with clips or brackets to prevent damage from vibration.
A flexible section must be included between the body and suspension to allow for steering and suspension movement.
These flexible lines are made of reinforced tubing, to protect them from objects that could be thrown by the tires.
In some vehicles, the brake lines are inside the vehicle to protect them from corrosion.
Background
For many years the tubing in automotive brake systems has been manufactured from low-carbon steel. Although coating composition has changed since the original hot-dip lead-tin coatings were used, coating flaws remain a problem. Current aluminum-zinc coatings and added polyvinylfloride coatings are still inadequate to totally protect the steel tube.
Brake Lines
The brake fluid travels from the master cylinder to the wheels through a series of steel tubes and reinforced rubber hoses. Rubber hoses are only used in places that require flexibility, such as at the front wheels, which move up and down as well as steer. The rest of the system uses non-corrosive seamless steel tubing with special fittings at all attachment points. If a steel line requires a repair, the best procedure is to replace the complete line. If this is not practical, a line can be repaired using special splice fittings that are made for brake system repair. You must never use brass "compression" fittings or copper tubing to repair a brake system. They are dangerous and illegal.
Source: CDX Global & Wikipedia - en.wikipedia.org
