Under normal conditions, to engage the parking brakes, the driver must first:

Engaging the parking brakes correctly is crucial for safety and stability when the vehicle is stationary. Under normal conditions, ensuring that the air brake system is fully pressurized is essential. This is because the parking brakes in air brake systems typically rely on air pressure to function properly.

When the system is fully pressurized, it allows the parking brake to engage effectively, ensuring that the truck remains securely stopped. If the air pressure is insufficient, the parking brakes may not engage correctly, leading to potential safety hazards such as the risk of the vehicle rolling away.

In contrast, turning off the engine, letting air out of the system, or checking brake fluid levels are not necessary prerequisites for engaging the parking brakes. Turning off the engine may be done after securing the vehicle, and while maintaining proper fluid levels is important for overall brake function, it is not a direct step in the process of engaging the parking brakes. Therefore, ensuring the air brake system is fully pressurized is the key process in this context.