Air brakes, hydraulic brakes and brake system checks
This section covers air and hydraulic brake systems, pre-trip checks and safe use on heavy vehicles. The exam tests whether you understand pressure, leaks, fade and the difference between a normal pedal and a dangerous one.
Core rules
Air-brake systems use a compressor, reservoirs, valves and brake chambers. Drain air tanks daily to remove moisture, check that pressure builds to the governed range, and listen for leaks with the engine off. A low-pressure warning buzzer or lamp means stop; do not drive until pressure is restored. Check slack adjusters and pushrods for excessive travel. Hydraulic systems need the correct fluid level, no leaks and a firm pedal; a spongy or sinking pedal is a defect. Both systems depend on correct adjustment and cooling. On long descents, select a low gear and use engine braking; do not ride the brakes, which causes fade.
Worked example
After a mountain descent, the brake pedal feels soft and travels further. The likely cause is overheated brake fluid or air in the hydraulic system, not normal wear. The safe response is to stop in a safe place, let the brakes cool, check for leaks and fluid level, and not continue until the fault is diagnosed. Pumping the pedal to “build it up” masks the problem.
Traps
Thinking a low-pressure warning can be ignored if the truck still stops; draining tanks only at service time; confusing slack-adjuster travel with normal movement; and using the foot brake continuously downhill instead of a low gear.