CDL General Knowledge TestCombination Vehicles (if applicable)Hard
What is the correct procedure to test the trailer service brakes using the trailer hand valve?
- ASet the tractor parking brakes, release the trailer parking brakes, and then apply the hand valve.
- BRelease all parking brakes, drive at highway speed, and then apply the hand valve firmly.
- CPush in both the tractor and trailer parking brake knobs, then apply the hand valve while driving slowly.
- DSet the tractor parking brakes, push in the trailer air supply knob, and then gently pull forward while applying the hand valve.
Show answer & explanationAnswer & explanation
Correct answer: D. Set the tractor parking brakes, push in the trailer air supply knob, and then gently pull forward while applying the hand valve.
To test the trailer service brakes, you first secure the tractor, release the trailer parking brakes (by pushing in the red knob), and then, while gently pulling forward, apply the trailer hand valve to feel if the trailer brakes are engaging properly.
Why the other options are wrong
- A. Releasing the trailer parking brakes is correct, but the test requires moving the vehicle slightly.
- B. Testing at highway speed is extremely dangerous and not part of a pre-trip inspection. All parking brakes released is also unsafe for this test.
- C. Driving with both parking brakes released and then applying the hand valve only tests the trailer brakes, but it's not the full, safe procedure. You need to ensure the tractor won't roll.
Trailer Service Brake Test
Procedure to verify that the trailer's service brakes are functioning correctly using the hand valve.
- Performed during pre-trip inspection.
- Tractor brakes set, trailer brakes released.
- Involves a gentle tug while applying hand valve.
Memory trick: Tractor set, trailer free, hand valve tests for you and me.