What happens when pressure is lost in the supply (emergency) line?

When pressure is lost in the supply (emergency) line, the trailer does not receive the necessary air pressure to engage its brakes, which results in no brakes being applied on the trailer. This situation is critical in a vehicle with an air brake system designed to remain safe under failure conditions. The purpose of the emergency line is primarily to provide a fail-safe mechanism; if the pressure drops to a certain level, the brakes should automatically apply to prevent an uncontrolled roll, but primarily on the trailer.

In this scenario, the correct outcome is that the trailer will have no brakes applied due to the loss of pressure. It is important to ensure the air system is functioning correctly to maintain braking effectiveness and safety.