Commercial Driver’s License (CDL) Practice Exam 2025 – All-in-One Guide to Master Your CDL Test!

The correct answer is the spring brakes. Spring brakes are a crucial safety feature in commercial vehicles equipped with air braking systems. Their primary function is to prevent the vehicle from rolling when the air pressure in the braking system drops below a specified level, which could indicate a failure or leak in the system.

Spring brakes utilize powerful springs that are held in a compressed state by air pressure while the vehicle is in operation. When the air pressure falls to a predetermined level, usually due to a malfunction or leak, the springs automatically engage the brakes to slow down or stop the vehicle. This mechanism serves as a fail-safe to enhance safety, particularly in emergencies where maintaining control of the vehicle is essential.

In contrast, the other options do not specifically address the scenario of low air pressure. A redundant brake line may help in preventing brake system failure by providing an alternative route for air, but it does not actively engage the brakes in response to low pressure. Brake shoe adjustment pertains to the maintenance of the braking system to ensure responsiveness but does not engage brakes automatically. The Electronic Stability Program is designed to enhance vehicle control and prevent skidding, but it does not function as a backup brake system in response to low air pressure.