One of the biggest enemies of your car's brakes is heat. Think about the fact that the same amount of work is performed to slow your car down as is performed to speed it up. That means that you produce a lot of heat going down hills. Most engines have coolant, a water pump, and a radiator to dissipate the heat of making the car go. The brake system has to do the same job with fewer tools.
The main way to get rid of heat is for the surface of the disks to absorb it and spread it out through the metal body of the disk and from there to the air rushing by the disk. Normally this system works fine. If the brakes are applied steadily over a period of time, though, the disk has no chance to get rid of the heat. If the heat builds up long enough, the brakes get hot enough to be damaged and then rendered ineffective. "Brake fade" means that the brake pedal feels fine but there is no longer adequate friction between the pad and disk. If the brake fluid reaches the boiling point, some of the fluid or contaminants (like water) mixed with the fluid boil and turn to gas. Unlike liquids, gases compress easily. The formerly firm brake pedal gets mushy or goes to the floor.
To avoid overheating the brakes, the driver should make sure to have the engine do some of the braking when going down long hills. That means that the transmission should be shifted into a gear that lets you take your foot off the brake pedal a fair amount of the time, thereby giving the brakes a chance to cool.
Another precaution you can take to prevent brake problems connected with overheating is to keep your brake fluid clean and pure. Your shop can help you do this by flushing the brake lines or by periodically replacing some or all of the brake fluid. | 
