Missouri CDL Practice Exam 2025 – Your All-In-One Guide to Certification Success!

Question: 1 / 400

When should the service line be opened?

Before starting the truck

After checking the systems

After the emergency line is closed

The service line should be opened after the emergency line is closed to ensure a proper sequence in establishing connections between the trailer and the towing vehicle. This is critical for maintaining control and safety during the coupling process. The emergency line is responsible for the trailer’s braking system, and closing it before opening the service line helps to prevent accidental brake activations or air leaks that could cause a safety hazard.

By completing these actions in the correct order, the driver ensures that the trailer and the vehicle are secured and that all systems are properly engaged without compromising safety. Establishing this protocol is essential, especially when managing air brake systems in commercial vehicles where missteps can lead to malfunction or accidents.

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During coupling

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