What component senses the reduction in pressure during a service brake application?

The control valve is responsible for sensing the reduction in pressure during a service brake application. When the engineer initiates a service brake application by moving the brake handle, the control valve responds by allowing air to flow from the main reservoir into the brake pipe, which causes a drop in pressure. This drop in pressure is crucial for the operation of the braking system, as it signals the brakes to engage.

The control valve's role is critical because it interprets the pressure changes and ensures that the appropriate amount of air is released to apply the brakes effectively. This mechanism allows for a controlled and smooth braking action, which is essential for maintaining safety and stability while the train is in motion. Other components, although they play significant roles in the overall braking system, do not perform the function of sensing pressure changes during a service brake application as directly as the control valve does.