What is the purpose of the service line in a combination vehicle?

The service line in a combination vehicle is essential for the braking system's functionality. It carries air to a relay valve, which then controls the brakes on the trailer. This air supply allows for the proper application and release of the trailer's brakes, ensuring that they respond effectively to the driver's commands through the vehicle's braking system.

This design is crucial for safety and control while operating large trucks or combination vehicles, as it ensures synchronized braking between the tractor and the trailer. The relay valve maximizes braking efficiency and responsiveness, which is vital when driving large vehicles that require longer stopping distances. The other options do not pertain to the function of the service line; they refer to power and connections that are not related to the braking operation in combination vehicles.