Question 5 of 140

What is the purpose of the supply (emergency) air line running between a tractor and trailer?

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It charges the trailer's air tanks and, if it breaks, triggers the trailer's emergency brakes
It only controls the trailer's turn signals
It is used solely to sound the trailer's horn, not to release the trailer's brakes in an emergency
It only powers the trailer's marker lights

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Correct answer to question 5 (Air Brakes): It charges the trailer's air tanks and, if it breaks, triggers the trailer's emergency brakes

Why

The supply line keeps the trailer's air tanks charged; if it separates or breaks, the resulting pressure loss automatically applies the trailer's emergency (spring) brakes.

Updated: November 2026

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What is the purpose of the supply (emergency) air line running between a tractor and trailer?

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