After connecting the air lines but before backing under the trailer you should:
Answer: B
Supply air to the trailer system, then pull out the air supply knob.
At this point, it is necessary to supply air to the trailer brake system by pulling out the air supply knob or moving the tractor protection valve control from the emergency to the normal position.
All choices
- AMake sure that the trailer brakes are off.
- BSupply air to the trailer system, then pull out the air supply knob.
- CWalk around the rig to be sure that is it clear.
Why the other answers are wrong
A. Make sure that the trailer brakes are off.
Trailer brakes are applied by supplying air, not turned off before backing under the trailer.
C. Walk around the rig to be sure that is it clear.
Walking around the rig is important but comes after supplying air to the trailer brake system.
CDL Combination Vehicles Test Info
Questions on Exam
20
Passing Score
80%
Time Limit
Typically 30 minutes; varies by testing location
The Combination Vehicles test content is federally standardized, so this question applies in every state. Age limits, fees, and testing procedures do vary — check your state's CDL requirements.
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More CDL Combination Vehicles Practice Questions
- Sudden turns or aggressive braking will have the greatest effect on which trailer?
- Which task is NOT performed during the tractor protection valve test?
- What is the purpose of the trailer air supply control?
- What is the most common cause of a tractor-trailer jackknife?
- There are two things that a driver can do to prevent a rollover. They are: (1) keep the cargo as close to the ground as possible and (2):