To get you up to speed on the infrastructure (very basically), your brake system is part mechanical--brake pads, rotors and the like--and part hydraulic--master cylinder, brake calipers and wheel cylinders in the case of shoed brakes. The brake fluid that is used to apply hydraulic pressure to the brakes at each wheel is contained within the brake line.
So when you press on the brake pedal pressure builds up in the master cylinder and that pressure is transmitted to the wheels through the brake lines. It is sort of like pressing the trigger on water gun except a brake system is a closed loop system. Leaks are not a good thing.
The brake lines are made of steel and are located under the vehicle which makes them susceptible to corrosion induced by road salt and other contaminants prevalent on the highway. Whether because of the extended length of time vehicles are staying on the road or the chemicals applied to assist winter driving, brake line failures are happening more often.
In April of 2010 the National Highway Traffic Safety Administration (NHTSA) opened a Preliminary Evaluation to investigate these failures in C&K Series trucks produced from the 1999 to 2003 model years. In a letter to General Motors, NHTSA said there had been 197 reports of brake line failure resulting in three crashes.
The trouble with sudden brake line failure is that there is rarely a warning. If the line is “weeping” an alert operator may become suspicious when brake fluid has to be added or the brake warning light on the dash lights up. At this point, you should have all the lines inspected and check for leaks at each wheel and around the perimeter of the vehicle. Brake fluid is clear and oily but will appear black as it discolors the pavement.
The lines are often hidden from the view of even a vigilant technician. There is no way to test the integrity of brake lines and they are too expensive to replace on a preventative maintenance basis. The same reason that makes them hard to evaluate--their inaccessibility--makes them labor intensive to replace. In some applications, brake line replacement jobs can cost more than $1,000.
If a brake line has ruptured pumping the brakes may not help. The National Safety Council's detailed instructions for handling your car when its brakes fail are available at link listed below. The first thing to remember is to try not to panic.
[NHTSA & National Safety Council]