Safe Driving Practices with Air Brakes
Air brakes stop a heavy vehicle, but not the way hydraulic brakes do. Driving questions test following distance, downgrade technique, and what changes when brakes get hot or wet.
Stopping distance
Total stopping distance is perception distance plus reaction distance plus brake lag plus braking distance. At 55 mph, perception and reaction are about 32 feet each and air brake lag is about 32 feet, before the brakes even begin to slow you. That is why a loaded tractor-trailer needs roughly 400 feet or more to stop at highway speed.
Downgrades and heat
Select a gear low enough that the engine holds the speed before you start down. Use firm, steady brake applications, then release, letting the engine do the work. Never fan or ride the brakes: continuous light application overheats drums, fades the linings, and lengthens stopping distance.
Wet, cold, and slick conditions
Water on the linings cuts friction; after a water crossing, apply the brakes lightly while moving to dry them. On ice and snow, brake gently and early, and let ABS do its job.
Worked example
You crest a 6% grade at 55 mph in high gear and speed climbs to 65. Reasoning: too fast, too high a gear. Brake firmly to bring speed down, then downshift; the correct move was selecting a low gear at the top.
Traps
Thinking air brakes apply instantly; pumping to restore pressure; using the parking brake to slow down on a grade.