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CDL · General

During the air brake check on a combination vehicle, how do you test the trailer emergency brakes?

Correct answer

Charge the system, then with the tractor parking brake off, pull out the trailer air supply valve — the trailer spring brakes should apply (then test by gently pulling forward to confirm they hold)

  1. A Skip the test
  2. B Charge the system, then with the tractor parking brake off, pull out the trailer air supply valve — the trailer spring brakes should apply (then test by gently pulling forward to confirm they hold)
  3. C Drive at high speed
  4. D Honk the horn

Why this is the answer

To test the TRAILER EMERGENCY (spring) BRAKES on a combination: charge the air system fully; then (with the vehicle stopped) PULL OUT the trailer air supply valve (or the system simulates air loss) — this should cause the trailer SPRING BRAKES to APPLY; gently try to pull forward with the tractor to confirm the trailer brakes HOLD. CDL combination. This verifies the fail-safe works (loss of air to the trailer applies its emergency brakes). Also test the trailer SERVICE BRAKES: push the supply valve in (charge), move forward slowly (~5 mph), and apply the trailer hand valve (trolley valve) — the trailer should brake. The combination air brake check includes testing both trailer service and emergency brakes plus the tractor protection valve. Knowing how to test the trailer emergency brakes is commonly tested.
Source: FMCSA CDL Manual — Combination, Trailer Emergency Brake Test

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