At what pressure will the spring park brakes start dragging in the event of a failure in the park/emergency circuit?

Answer: C — 60 psi

The correct answer indicates that the spring park brakes will begin to drag at 60 psi during a failure of the park/emergency circuit. This is an important safety feature designed to prevent unintended vehicle movement when air pressure drops below a crucial level. When the air pressure in the system falls to this threshold, the brake springs that are supposed to engage the brakes may not be adequately released or may not stay released as intended, leading to the brakes partially engaging or dragging. This situation can cause complications, such as increased tire wear, overheating, and more importantly, potential loss of control of the vehicle. Understanding the specific pressure at which this phenomenon occurs is vital for operators to monitor and maintain their air brake systems effectively. By ensuring that pressure levels are above this point, the risk of brake drag and associated issues can be minimized, thereby ensuring safer operation of the vehicle.