Air Brakes and Brake Systems on Combination Vehicles
What this domain covers
Air-brake questions on the combination test focus on how tractor and trailer air systems work together: supply, protection, emergency braking, and warning. You need to know what happens when pressure falls and why the trailer must never be left without air.
Core rules
- The compressor charges the tractor and trailer through the supply line. The tractor protection valve closes the trailer supply if pressure drops, applying the trailer emergency/spring brakes.
- The trailer has a relay valve and spring brakes. If the red line is disconnected or breaks, the trailer brakes should set automatically.
- The low-air warning must activate before 60 psi; spring brakes normally apply between 20 and 45 psi.
- Do not fan the brakes. Use engine braking or a lower gear on long downgrades and apply brakes firmly only when needed.
- Drain air tanks daily to remove moisture; in cold weather, moisture can freeze lines and valves.
- The trailer hand valve is for testing, not parking.
Worked example
You hear the low-air warning at 55 psi but continue to a scale. At 30 psi the spring brakes begin to apply. That is not a brake fade problem; it is normal protection. The correct response is to stop safely as soon as the warning sounds, not to pump the pedal.
Traps that cost marks
Thinking the trailer hand valve is a parking brake; ignoring the low-air warning; failing to drain tanks; testing with the engine running; and assuming a trailer breakaway will leave the trailer free to roll. The red line is designed to set the trailer brakes.