Normal Operation and Safe Driving with Air Brakes
This domain tests how to use air brakes without overheating them or running out of air: braking technique on grades, brake lag, correct use of the parking and trailer valves, and speed control.
Braking technique
- For normal stops, use a steady, light-to-moderate foot-valve application and release smoothly.
- Air brakes have lag: at 55 mph, roughly 32 feet is travelled before the brakes begin to apply, so following distance must be generous.
- Do not “ride” the brakes. Continuous light application on a long downgrade overheats drums and linings, causing brake fade - the pedal feels normal but stopping power disappears.
- Instead, shift into a low gear before the grade and use snubbing: apply firmly to reduce speed about 5–10 mph, release, let the brakes cool, and repeat.
- Use the same gear going down as you needed going up.
Air management
Every application uses reservoir air. Fanning the brakes repeatedly can exhaust the system faster than the compressor replenishes it. Watch the supply gauge on long grades; if it falls toward the warning range, stop and let it build.
Valves
The yellow parking knob applies spring brakes for parking - never for a normal stop except in an emergency. The trailer hand valve (spike) is for testing trailer brakes only; it is not a parking brake and must not be used with the foot brake, because compounding can damage the system.
Worked example
A loaded tractor-trailer starts down a 6% grade at 55 mph in high gear. The driver brakes lightly and continuously; after a mile the pedal feels firm but the truck will not slow. Diagnosis: brake fade from heat. Correct action: shift to a lower gear, release the brakes to cool, use snubbing, and if necessary use an escape ramp.
Traps
Believing “fanning” is a cooling technique; using the trailer hand valve to hold a parked vehicle; and waiting until the supply gauge is low before reacting.