CDL Vehicle Combination Practice Test

The service air line is the correct answer because it directly connects the tractor's braking system to the trailer's braking system. When you apply the foot brake in a combination vehicle, it sends air pressure through the service air line, which activates the trailer brakes, providing braking power to the entire combination vehicle. This linkage ensures that the brakes respond effectively, allowing for smoother stops and better control.

The other options, while related to the braking system or vehicle operation, do not directly respond to the operation of the foot brake. The emergency air line is used for the emergency brake system and for maintaining the trailer's brakes in case of a failure in the service air system. The trailer brake light is activated when the brakes are engaged, but it does not control the braking mechanism itself. Lastly, the tractor throttle manages the engine speed and is not influenced by the foot brake. Understanding this direct relationship between the foot brake and the service air line is crucial for safe operation of combination vehicles.