What is the most important safety check before driving a vehicle with air brakes?

The brake application and reapplication test is crucial because it directly assesses the functionality and effectiveness of the air brake system before the vehicle is put into operation. This safety check ensures that the brakes engage properly and release, confirming that the air brakes are functioning as intended. Proper operation of the air brakes is vital for the safe stopping and control of the vehicle, especially given the unique characteristics of air brake systems, where pressure plays a key role in their operation.

Ensuring the brakes work correctly helps prevent accidents due to brake failure, which can have catastrophic results. Conducting this test allows drivers to identify any potential issues, such as air leaks or problems with the brake chambers, before they become serious.

In contrast, while tire pressure is important for overall vehicle safety and performance, it does not directly affect the air brake system's efficiency. The engine start-up procedure is essential for ensuring that the vehicle is ready to operate, but it also does not provide specific information about the air brakes' condition. Lastly, the alignment of the steering wheel, while necessary for driveability, is not related to the braking system's performance. Therefore, prioritizing the brake application and reapplication test is imperative when it comes to ensuring air brake safety.