Which of the following is essential when engaging the trailer emergency brake?

Engaging the trailer emergency brake requires ensuring that the air supply control is released. This is crucial because the air supply control regulates the flow of air to the trailer's brake system. When this control is properly released, it allows the air to reach the trailer's brakes, enabling them to function correctly. If the air supply is not released, the trailer brakes will not engage, potentially leading to dangerous situations, especially if the truck is parked or if there's a need to stop the vehicle in an emergency.

In contrast, while monitoring air pressure is important for overall safety and performance, and pumping the brakes slowly can be a technique used for various braking scenarios, they do not specifically address the immediate requirement of engaging the trailer emergency brake effectively. Similarly, performing a visual inspection prior to driving can be a good practice for checking the overall condition of the vehicle, but it does not directly pertain to the procedure for engaging the trailer emergency brake. Ensuring the air supply control is released is the key action that must be taken to properly activate the emergency braking system for the trailer.