Using Air Brakes While Driving
Braking technique
This domain tests how you use air brakes on the road, especially on grades and in traffic. Air brakes do not apply instantly: at highway speed there is a lag of roughly 30 feet before the brakes begin to work, so following distance and smooth control matter more than with hydraulic brakes.
- Apply the foot valve progressively and release smoothly. Do not fan or pump air brakes in normal stopping; each application uses reservoir air.
- On a downgrade, choose a low gear before the descent and use snub braking: brake firmly to reduce speed, release, let speed rebuild, and repeat. Do not ride the brakes.
- Watch the pressure gauge. If pressure falls toward the warning range, stop and find the cause. Do not continue a descent hoping it recovers.
- The parking brake is for parking after stopping, not for routine speed control. Use engine braking and transmission gearing as primary controls.
- With ABS, brake normally and steer. Do not pump the pedal; an ABS light means the vehicle may have normal brakes without ABS.
Worked example
A loaded tractor-trailer starts a long 6 percent grade at 35 mph. The driver selects a lower gear before the crest, brakes down to 25 mph, releases, and repeats when speed reaches 35 mph. Pressure stays above 100 psi. If the gauge fell below 60 psi, the warning would sound and spring brakes could apply; the correct response is to stop safely, not to fan the brakes.
Traps
Coasting in neutral removes engine braking and is illegal in many situations. Riding the brakes overheats drums and causes fade. Using the parking brake at speed can lock wheels. Do not treat an ABS warning as a total brake failure.