Normal Driving with Air Brakes (Stopping, Downgrades, Brake Fade)
Normal Driving with Air Brakes (Stopping, Downgrades, Brake Fade)
This domain tests smooth, controlled use of air brakes: stopping distance, brake lag, downgrade technique, and preventing fade. The exam expects you to choose a gear and a speed before the grade, not to fix a hot brake halfway down.
Core rules
- Air brakes have lag. After you press the pedal, air must travel to the chambers and pushrods must move. At 55 mph, brake lag alone can add roughly 32 feet before the brakes begin to work.
- Total stopping distance includes perception distance, reaction distance, brake lag, and braking distance. Air brakes do not stop as quickly as hydraulic brakes.
- Before a downgrade, select a gear low enough that engine braking holds speed. Use the service brake firmly to bring speed down, then release; repeat. Do not ride the brakes.
- Brake fade occurs when drums and linings overheat, reducing friction. Signs include a low, soft pedal, a burning smell, and needing more pressure for the same stop.
- If fade begins, stop, cool the brakes, and check adjustment. Do not continue down the grade relying on a fading brake.
Worked example
A loaded truck enters a 6 percent downgrade at 55 mph in high gear. The driver rides the service brake, and by the midpoint the pedal feels soft and the truck speeds up. The mistake was entering too fast and too high a gear. The fix is to slow before the crest, shift to a lower gear, and use controlled applications. Once fade is advanced, the driver should find a safe place to stop and let the brakes cool.
Traps
Do not coast in neutral on a downgrade. Do not use the parking brake to control speed; spring brakes can lock wheels. Do not think a soft pedal after hard use means the brakes are “warmed up and better.”