Illinois CDL Practice Exam

What do the blue and red hose couplers ("Glad Hands") represent in a trailer's air lines system?

1. Temperature indicators

2. Service line and emergency line indicators

3. Brake fluid types

4. Engine oil levels

The correct answer is: Service line and emergency line indicators

The blue and red hose couplers, commonly referred to as "Glad Hands," are crucial components in a trailer's air lines system, serving to differentiate between two primary functions: the service line and the emergency line. The service line, indicated by the blue coupler, is responsible for supplying air to the trailer’s brake system during normal operation, enabling the brakes to engage as needed while the vehicle is in motion. The emergency line, marked by the red coupler, plays a pivotal role in ensuring that the trailer’s brakes can activate in the event of a disconnect from the tractor or a loss of air pressure, thereby implementing emergency braking to prevent accidents and loss of control. This color-coding system is standardized for safety and ease of use, ensuring that drivers and maintenance personnel can quickly identify the purpose of each line. Proper understanding and correct connection of these air lines are essential for the safe operation of trailer brakes, emphasizing the importance of the blue and red couplers in a truck-trailer system.