Required for every applicant

Free CDL Combination Vehicles practice test 2026

Needed for Class A tractor-trailer, including coupling and uncoupling order.

A sample question

One example, answer shown, with the handbook section it comes from.

Why should you avoid using the trailer hand valve while driving?

It can cause a trailer skid or jackknife(correct answer)
It drains the air tanks too quickly
It disables the tractor's service brakes
It applies the brakes too gradually to be useful

The hand valve applies only the trailer brakes. Using it while moving — especially on a slippery surface — can lock the trailer wheels and start a trailer skid or jackknife.

Handbook section 6.2

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