During normal driving, what usually holds back the spring brakes?

The correct response highlights that air pressure is the primary force that holds back the spring brakes during normal driving. In air brake systems, the spring brakes are designed to engage automatically in case of an air pressure loss, which acts as a safety feature. Under typical operating conditions, these brakes are held in the released position by compressed air that maintains the system pressure. When the driver applies the brakes, the air pressure decreases, allowing the spring brakes to work effectively as a backup and engage when necessary to bring the vehicle to a stop.

Understanding this principle is crucial for safe vehicle operation, as it emphasizes the importance of maintaining adequate air pressure within the brake system. If there is a failure in maintaining this pressure, the spring brakes could inadvertently activate, leading to unexpected braking action. This knowledge helps a driver effectively monitor and respond to their braking system's performance, ensuring they adhere to safety protocols while on the road.