Air Lines and Brake System Checks
What the exam checks
This domain tests air-line routing, glad-hand connections, leak inspection, and the brake tests you must perform on a multi-trailer combination. The examiner wants to see that you understand how the tractor protection valve, trailer air supply, and dolly lines affect all trailers.
Core rules
Connect the service line (blue) to service and the emergency line (red) to emergency; crossed glad hands can release trailer brakes or prevent charging. Inspect coupler seals, lock rings, and hoses for cuts, bulges, and chafing. Secure lines so they do not drag or rub the dolly. Charge the system, then shut the engine off and check for static leaks with brakes released and applied. Listen for leaks at glad hands and dolly connections. Test that the low-pressure warning activates before pressure falls too low and that spring brakes set. With the tractor protection valve in normal position, each trailer should charge and release its brakes; with the valve closed or emergency line disconnected, trailer brakes should apply.
Worked example
A driver hears a leak at the dolly's emergency glad hand. Because the emergency line supplies the trailer air tanks, a leak can drain the dolly and rear trailer, causing drag or brake loss. The driver should replace the coupler seal, reconnect, recharge, and repeat the leak test before moving.
Traps that cost marks
Do not check only the tractor gauge; leaks at the dolly affect the rear trailer. Do not leave glad hands uncoupled or dangling. Do not assume a low-pressure warning is optional. Do not test brakes with the engine running and call it a leak test.