If the service line separates while driving a combination vehicle but the emergency line remains intact, what usually occurs immediately?

When the service line separates while driving a combination vehicle but the emergency line remains intact, nothing happens immediately until the brakes are applied. The service line is responsible for transmitting air pressure to the trailer's service brakes, which means that without it intact, the trailer's brakes do not engage automatically. This condition does not activate the trailer brakes because they are dependent on the service line for air pressure.

The emergency line serves a different purpose; it is meant to activate the trailer brakes in the event of a complete disconnection. Therefore, when the service line fails but the emergency line remains connected, the driver will not experience any immediate braking action or loss of control of the vehicle until the brakes are manually applied. This understanding is critical for drivers of combination vehicles because it highlights the importance of recognizing the signs of air system failures and implementing appropriate responses to maintain control of the vehicle.