Michigan CDL - Air Brakes Practice Exam

When should the spring brakes automatically engage in a vehicle with air brakes?

1. When air pressure exceeds 60 psi

2. When air pressure falls below 45 psi

3. When air pressure is between 30 and 40 psi

4. When the engine is turned off

The correct answer is: When air pressure falls below 45 psi

The spring brakes should automatically engage when air pressure falls below 45 psi. This automatic activation of the spring brakes is an essential safety feature in vehicles equipped with air brake systems. It ensures that the vehicle comes to a stop in the event of a significant loss of air pressure, indicating that the air supply is insufficient to effectively operate the air-braking system. The 45 psi threshold is established to provide a safeguard against brake failure. When the pressure drops to this level, the system recognizes that there may be a malfunction or potential risk of brake loss and engages the spring brakes. This mechanism helps prevent accidents that could occur if a driver were to lose braking capability while on the road. In contrast, the other options do not specify the proper operational thresholds for the engagement of the spring brakes. For instance, exceeding 60 psi does not trigger any response regarding the spring brakes; that situation indicates the air supply is sufficient. Similarly, air pressure between 30 and 40 psi is also too low to ensure the proceeding safety measures that the spring brakes provide. Lastly, simply turning off the engine does not activate the spring brakes; it is the air pressure that dictates the engagement. Thus, the reliable connection between low air pressure and the activation of spring brakes is