CDL Practice Question #171

The trailer hand valve should be used:

Only to test the trailer brakes; never to slow the rig in normal driving
To slow the rig in all conditions
To brake on slick roads
Continuously on long downgrades

Correct. Use the trailer hand valve only for testing trailer brakes. Using it to slow the rig can cause the trailer to skid or jackknife.

Why this question matters

This question is one of 55 in our Combination Vehicles bank. The reasoning above comes directly from the AAMVA CDL Manual, the source document used by every U.S. state DMV. Memorizing the underlying rule — not just the letter answer — is what separates passing on the first attempt from a return trip and another fee.

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Full Combination Vehicles practice test
Combination Vehicles study guide
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