What is brake fade, and what causes it?

Brake fade is the loss of stopping power that occurs when brakes overheat. Friction between the brake linings and drums or rotors generates heat, and as the brakes get hotter, they lose grip. On a long downgrade where the brakes are applied continuously to control speed, heat builds up faster than it can dissipate, and the brakes progressively lose effectiveness. Eventually they can lose so much grip that the vehicle accelerates uncontrollably even with the brake pedal fully depressed. Prevention: select a low enough gear that engine braking does most of the work on long downgrades, and use the service brakes only intermittently to bleed off speed. The 'snub braking' technique (brake hard to drop 5 mph, then release until speed builds back up, then snub again) keeps brakes cooler than continuous light braking.