What happens to the air pressure when brakes are applied?

Answer: A — Air pressure in the system decreases as it is used to activate the brakes

When brakes are applied in an air brake system, air pressure in the system decreases because the compressed air is released to activate the brake chambers. This process requires the air to push the brake components into action, thus utilizing the stored air pressure. As the brakes engage, the pressure needed to push the brake pistons consumes some of the air stored in the system, leading to a reduction in overall pressure. This is a fundamental aspect of how air brake systems function, where the application of brakes directly correlates to a drop in pressure as part of the braking process. The other options do not accurately reflect the mechanical principles at play; for instance, air pressure does not remain constant when brakes are applied, nor does it increase due to compression. Additionally, air pressure remains crucial to the functioning of the brakes rather than becoming irrelevant once engaged.