CDL Vehicle Combination Practice Test

When should the trailer air supply control automatically close?

1. When air pressure fails to between 20 and 45 psi

2. When the trailer is disconnected

3. After the coupling process

4. When the brakes are applied

The correct answer is: When air pressure fails to between 20 and 45 psi

The trailer air supply control is designed to enhance safety and prevent unintended movement of the trailer when conditions are not suitable. It automatically closes when air pressure in the system falls between 20 and 45 psi. This low-pressure setting indicates a potential failure in the air braking system, which could lead to a loss of braking capability or control over the trailer. By closing at this pressure threshold, the system helps ensure that the trailer's brakes remain engaged, thereby preventing accidents or runaways. This mechanism is crucial for maintaining safety standards and ensuring that drivers are aware of potential issues with their air supply system before they can operate the vehicle under unsafe conditions. The automatic closure serves as an important indicator that the driver needs to address a serious issue within the air supply system.