You are driving a combination vehicle when the trailer breaks away, pulling apart both air lines. You expect the trailer brakes to come on and:
Answer: A
The tractor protection valve to close.
If all the air in the trailer is lost to a leak, the emergency brakes will be activated and the tractor protection valve will close.
All choices
- AThe tractor protection valve to close.
- BThe trailer supply valve to stay open.
- CThe tractor to lose air pressure.
Why the other answers are wrong
B. The trailer supply valve to stay open.
Choice B is wrong because the trailer supply valve will close, not stay open, when air lines break apart.
C. The tractor to lose air pressure.
Choice C is wrong because the tractor does not lose air pressure; instead, the tractor protection valve closes to protect the 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
- Should the trailer hand brake be applied when a trailer starts to jackknife?
- Glad hands are usually connecting which of these?
- Why should you lock the tractor glad hands to each other (or dummy couplers) when you are not towing a trailer?
- After connecting the air lines but before backing under the trailer you should:
- The crack-the-whip effect is related to: