Vehicle Combination and Air Brakes Overview
Coupling
Check the fifth wheel, kingpin, landing gear, air lines, electrical cord, and trailer brakes. After coupling: tug test, visual check, raise landing gear, connect air lines, check trailer brake lights.
Combination behavior
Tractor-trailers off-track, amplify rearward movement, and can surge. Bobtail, tractor only, brakes differently and can lock rear wheels. Use the trailer hand valve only for testing, not parking.
Air brakes
The system has a compressor, governor, tanks, and dual circuits. Test for leaks, check the low-pressure warning, and confirm spring brakes apply. Don't fan the brakes.
Worked example
After coupling, a driver pulls forward without a tug test. Reasoning: the kingpin may not be locked. A tug test with the trailer brakes set reveals a bad coupling before it becomes a dropped trailer.
Traps
Forgetting the tug test; leaving landing gear down; crossing glad hands; using the hand valve as a parking brake.