Air Brakes and Brake System Checks
Air brake knowledge is the most heavily tested area, so learn the sequence cold. Know the supply pressure range (governor cut-out around 120-125 psi, cut-in around 100 psi), the low air warning (must activate by 60 psi), and the spring brake pop-out (typically 20-45 psi). Know that the tractor protection valve closes to keep the tractor's air from escaping if a trailer breaks away, and that the trailer air supply knob (red) must be pushed in to charge the trailer system.
A complete check has three parts: static leak test (engine off, fully charged, no drop greater than 3 psi in one minute for a single vehicle, 4 psi for a combination, with brakes released; no more than 3 psi per minute with the brakes applied), applied leakage test and low pressure warning/spring brake test, then the pull test to confirm the trailer brakes hold. Expect trap questions about the order of steps, about whether the engine should be running (it should be off for leak tests), and about what a rapid loss of pressure means (do not drive).
Worked example: with a fully charged system and the engine off, pressure falls from 120 to 115 psi in one minute with brakes released. That is a 5 psi drop - above the 4 psi combination limit, so the vehicle fails the static leak test and must be repaired before driving.
Remember: never fan the brakes on a long downgrade; use a low gear and stab braking.