CDL General Knowledge TestCombination Vehicles (if applicable)Medium
What is the recommended procedure for checking the trailer's emergency brakes during a pre-trip inspection?
- AApply the foot brake firmly and check for air leaks at the glad hands.
- BUse the trailer hand valve to apply the trailer brakes and then release the tractor parking brakes.
- CPush in the trailer air supply valve and ensure the trailer moves when trying to pull away.
- DPull out the trailer air supply valve and check if the trailer parking brakes hold.
Show answer & explanationAnswer & explanation
Correct answer: D. Pull out the trailer air supply valve and check if the trailer parking brakes hold.
To check the trailer's emergency brakes, you pull out the trailer air supply (red) knob, which dumps air from the emergency line and applies the trailer's spring (emergency) brakes. Then, you gently tug forward to ensure they hold.
Why the other options are wrong
- A. This checks for service brake leaks, not the function of the emergency brakes.
- B. Using the trailer hand valve checks the *service* brakes, not the emergency brakes. Releasing tractor brakes would test the tractor's service brakes in this scenario.
- C. Pushing in the trailer air supply valve *releases* the trailer brakes. The trailer should *not* move if the tractor brakes are set.
Trailer Emergency Brake Check
Procedure to verify the functionality of a combination vehicle's trailer emergency brakes.
- Uses the trailer air supply valve (red knob).
- Verifies spring brake application.
- Confirmed by a gentle tug test.
Memory trick: Red knob out, no roll about.