The spring-loaded brake has been applied on a truck which has broken down with engine failure. The air supply is exhausted. What effect does this have?

You answered correctly. On trucks with spring-loaded brakes, if the engine fails and the air supply is exhausted, the brake remains engaged because air pressure is required to release it. This means the spring-loaded brake can no longer be controlled by the handbrake valve, and to move or tow the vehicle, the emergency release device must be used to manually release the brake. There is no need to physically remove the springs; using the release device is the proper method. Understanding this ensures you know how truck braking systems work in emergencies and towing situations.