CDL Vehicle Combination Practice Test

Before backing a tractor under a semi-trailer, what should you connect first?

1. The electrical lines

2. The emergency and service air lines

3. The safety chains

4. The load bars

The correct answer is: The emergency and service air lines

Connecting the emergency and service air lines before backing a tractor under a semi-trailer is essential for safe and effective coupling. These air lines provide the necessary pneumatic connections that control the trailer's braking system. The emergency line ensures that if the connection is lost while driving, the trailer's brakes will engage automatically, preventing a runaway situation. The service line allows for normal braking operations while driving. This step is crucial because it makes the trailer operational and ensures that braking functionality is in place before any further steps, such as connecting the electrical lines or safety chains, are taken. Other connections, like the electrical lines or safety chains, can be done after ensuring that the air lines are properly connected. This order of operations prioritizes safety and functionality, making sure that the trailer is immediately ready to respond to braking needs once the tractor is backed in. The focus on the air lines first underscores the importance of having a functioning braking system as the primary concern in a vehicle combination setup.