What action should be taken during a full service brake application?

During a full service brake application, the primary goal is to reduce the brake pipe pressure in a controlled manner to engage the brakes effectively. The correct action is to reduce brake pipe pressures significantly. This reduction in pressure sends a signal to the braking system to activate and apply the brakes across the train, ensuring a smooth and effective deceleration.

The other options do not align with the purpose of a full service brake application. For instance, opening the service brake valve (which might imply a minor adjustment rather than a significant reduction in pressure) wouldn't achieve the necessary braking force needed in a full service situation. Initiating a small pressure increase is counterintuitive, as that would actually work against the function of applying the brakes. Lastly, equalizing all brake pressures is more relevant during a setup phase than a direct action for applying brakes since it’s about ensuring the brake system is balanced rather than actively applying the brakes. Thus, the focus should be on a significant reduction of brake pipe pressures to effectively engage the brakes during a full service application.