Air Brakes and Basic Control
Air brake knowledge is tested heavily because failures are catastrophic. Candidates must understand the components and their roles: the compressor builds pressure, the governor controls when the compressor loads and unloads, the reservoirs store air, the foot valve modulates application pressure, and the spring (parking) brakes apply automatically when pressure drops. The low-pressure warning must activate before pressure falls below the manufacturer's threshold (commonly around 60 psi), and the spring brakes should apply when pressure drops further. The leak-down test checks that pressure loss stays within limits with the engine off and brakes released, then applied.
Traps include confusing the roles of the governor and the compressor, and misunderstanding the supply versus service lines in a trailer. Another common trap is the cut-out/cut-in behaviour of the governor and what it means when pressure will not build. Questions also test the correct response to a low-pressure warning: stop safely, do not continue driving on the remaining pressure.
Rule of thumb: if the low-pressure warning sounds, treat it as an emergency - bring the vehicle to a safe stop and investigate. Worked example: with the engine off and brakes released, pressure drops 10 psi in one minute; with brakes applied it drops 15 psi. If the vehicle's specification allows 3 psi per minute released and 4 psi applied, both readings exceed the limit and the vehicle must be taken out of service for leak repair.