The simple act of stepping on a brake pedal can generate a tremendous amount of heat. Brakes rely on friction, and it's easy for drivers to forget that they are sitting in massive, fast-moving machines weighing thousands of pounds. Just slowing them down can cause brake pad temperatures to rise by hundreds of degrees.
"Stepping on a brake pedal can generate a tremendous amount of heat."
In most modern passenger cars, the widespread use of disc brakes on all four wheels has significantly improved heat management. The discs themselves are completely exposed to air, which keeps them cool even during heavy use. However, freight trucking fleets tend to rely on drum brakes. According to an article on Heavy Duty Trucking, fleets are interested in the stopping benefits of disc brakes, but have been slow to adopt them for cost reasons. As drum brakes are more susceptible to heat buildup, they are at a higher risk of malfunctioning if not properly maintained.
An article on The Brake Report explains that the temperature at a drum brake's engagement point – that is, where the shoes meet the drum – can get as high as 1,000 degrees Farenheit. Of course, that heat is then transferred to the cast-iron drum, which acts as a heat sink, and dissipated into the air. Normally, this works just fine. Even so, the news source notes that there are certain scenarios – such as heavy use or restricted air flow – that can cause temperatures to rise past operational norms and cause lining to degrade. The result is brake fade.
Not long ago, federal regulators implemented stricter stopping distance requirements for truck fleets. As most of these trucks still rely on drum brakes, it's important for a vehicle inspection service to make sure that their brakes remain functional.