Using Air Brakes While Driving
Smooth control
Air brakes are not instantaneous: air must travel from the reservoir to the chambers, so there is lag. Apply steady, progressive pressure and plan stops earlier. Do not fan the brakes; each application uses air, and fanning can drop pressure faster than the compressor can recover. On long downgrades, select a lower gear before the grade and use engine braking plus short, firm applications. Do not ride the brakes, which overheats drums and causes fade.
Trailers and parking
The trailer hand valve is for testing trailer brakes, not for parking or normal stopping. The parking brake is for parking; using it as a service brake can damage the mechanism and may not give controlled stopping. With ABS, press and hold the brake pedal during an emergency stop; do not pump.
Worked example
A loaded tractor-trailer starts a long downgrade at 55 mph in high gear. The driver uses repeated light applications; pressure falls to 60 psi and the low-pressure warning sounds. The correct response is to stop safely, select a lower gear, let the brakes cool, and resume at a lower speed with steady applications. Continuing to fan the brakes risks losing air and overheating.
Traps
Thinking air brakes apply instantly loses marks. Using the trailer hand valve as a parking brake is wrong. Riding the brake pedal downhill causes fade and air loss. Coasting in neutral downhill removes engine braking and control. Ignoring the low-pressure warning is dangerous.
Speed and space
Air-brake vehicles need longer stopping distances than cars. Increase following distance, brake early, and avoid excessive speed on grades.