Using Air Brakes While Driving
This domain tests how to use air brakes while driving: recognizing lag, controlling speed on grades, avoiding fade, and choosing the right brake controls.
Braking technique
Air brakes have lag. At 55 mph, the vehicle may travel about 32 feet before the brakes begin to apply, so following distance and early braking matter. Use the foot valve for normal stopping. The trailer hand valve applies only trailer brakes and can cause a jackknife; use it for testing, not routine stops. On a downgrade, slow down and select a low enough gear before the grade. Use engine braking and short, firm service brake applications rather than continuous light pressure. Continuous braking overheats drums and causes fade. Monitor the air gauge and keep pressure in the governed range.
Worked example
A loaded tractor-trailer starts down a 6% grade at 55 mph in high gear. The correct response is to slow before the grade, shift to a gear that holds speed, use engine braking, and make controlled service brake applications. Riding the brakes continuously is wrong because heat buildup can fade the brakes and leave too little stopping power.
Traps that cost marks
Do not coast in neutral to save brakes. Do not use the trailer hand valve for normal stopping. Do not wait until speed is too high before downshifting. Do not ignore the air pressure gauge on a long grade. Do not fan the brakes repeatedly in place of a proper low gear.