Although air brakes are more-or-less buletproof these, like all braking systems, have one failing: BRAKE FADE.
In the last thirty years air brakes have become more or less bullet proof. The reason I say that is because we put them on vehicles that carry our most precious cargo... school buses. You will not find a school bus manufactured in the last five years that is not equipped with air brakes. There all equipped with air brakes and if air brakes were unreliable, there's no way we would be putting them on school buses. So air brakes are reliable, however they have one weakness left. That weaknesses is brake fade.
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Brake fade is when you overheat the brake drums and they expand away from the shoes. The way that brake drums work is is that there is a shoe inside of the drum. When you push down on the brake pedal‚whether a hydraulic system or an air brake system‚ it forces the shoe out against the drum, creates friction, drag and brings the vehicle to a stop. In the process of pushing the shoes out against the drum, you create heat.
Heat Dissipated Into the Atmosphere
Often times there's enough time lapse between the brake applications that that heat is dissipated into the atmosphere, the same way that an air-cooled engine has fins on the side of the engine and the air passes over those fins and pulls that heat into the atmosphere. Brake drums work the same. The air passes over the drum, over the rim, and the heat is dissipated into the atmosphere. Now if you hold the brakes down too much on a heavy downhill, a big downhill rather, with a heavy load, you hold the brakes on all the way down you potentially heat them up too much where the heat cannot be dissipated into the atmosphere, the drum expands away from the shoes and no matter how hard you push down that brake pedal those shoes are not going to come in contact with the drum. It's called brake fade. If you put enough heat to the brake drum it can catch on fire and so too can the tire.
Now, one of the other things that's beginning to combat this is disc brakes. They are beginning to make inroads into the industry but the technology's just not quite there yet. The benefit of disc brakes is that they do not experience brake fade, and this is why we find them on high-end sports cars and motorcycles. Because when you heat them up, the rotor heats up and expands into the brake pads, not away from them. So therefore disc brakes work better when they're heated up and do not experience brake fade.
Brake fade is the last weakness of the air brake system. If you put too much pressure to the brakes you're going to heat them up. If you heat them up, no matter how hard you push on that brake pedal the shoes are not going to come in contact with the drum. Brake fade‚ it is required for an air brake endorsement and it is much more important when you're doing heavy downhill braking in British Columbia, West Virginia, and other places in North America that have big hills. So keep it in mind and know the dangers of brake fade. And when I advocate for downhill braking, is short, hard, intermittent brake applications - don't hold the brakes on for very long. You wanna have them off long enough that the air passes over the drums and is allowed to dissipate that heat into the atmosphere.