Heavy Vehicle Braking, Air Brakes and Speed Control
How the system works
Heavy vehicles use air-assisted braking, often with a dual circuit so a single failure does not remove all braking. Air pressure builds to an operating range; low pressure gives a warning and, at a lower threshold, a protection valve that limits further loss. Understand the park brake, service brake, secondary circuit and retarder or engine brake.
Using brakes well
Brake progressively and early. On long descents, select a low gear and use engine braking or a retarder to hold speed, applying the service brake in short applications rather than riding it continuously, which overheats and fades the brakes. Never coast in neutral. Allow for longer stopping distances when loaded, in rain, or on gravel.
Air brake checks
Know the daily checks: build pressure, listen for leaks with the engine off, confirm the low-pressure warning works, and test the park brake. Draining air tanks removes moisture and oil. A leaking system that loses pressure faster than specified must be reported.
Traps
Questions often describe a descent where the driver uses the footbrake all the way down; the correct answer is a low gear plus retarder. Another trap is ignoring a low-pressure warning as a sensor fault.
Rule of thumb
Select the gear you can descend in before the descent begins, and treat any pressure loss as a defect.
Worked example
On a long downhill you find the brakes fading. Correct: use a lower gear and retarder, reduce speed, and stop safely to let the brakes cool. Pumping the pedal harder is wrong.