What is 'brake fade' (zbehja e frenave) in heavy vehicle foundation brakes, and what causes it?

1. A condition where brake pads freeze to the drums in cold sub-zero weather
2. A loss of air pressure due to a faulty air compressor governor
3. A severe drop in friction coefficient and braking power caused by excessive heat buildup in friction linings and drums during prolonged continuous braking
4. The natural fading of the red paint on brake warning lights over time