2026 Oklahoma Combination CDL Practice Test 2

Describe what the trailer air supply control does.

It is a device used to keep the trailer behind the tractor. It is a six-sided yellow knob used to control the tractor protection valve. It is used to supply the trailer with air, shut the air off, and apply the trailer emergency brakes.

Correct Answer:

It is used to supply the trailer with air, shut the air off, and apply the trailer emergency brakes.

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