Final answer:
Crossing the air lines on a trailer without spring brakes will prevent the brakes from operating correctly, potentially leading to no braking force at the trailer wheels, ineffective stopping, and an increased risk of a runaway trailer. It's crucial to connect the air lines properly for safe operation.
Step-by-step explanation:
If the air lines are crossed when hooking up to an old trailer with no spring brakes, it can lead to a dangerous situation where the brakes may not function as intended. Normally, the blue line (service line) controls the brakes when the brake pedal is pressed, and the red line (supply/emergency line) charges the trailer's air tanks. When these lines are reversed, applying the brake pedal could potentially cause air to be supplied to the trailer air tanks instead of applying the brakes, leading to a lack of braking power.
This improper configuration can result in no braking force at the trailer wheels, preventing the trailer from stopping effectively when the brakes are applied. Moreover, if the trailer is equipped with a fail-safe system designed to apply the brakes in the event of a pressure loss, this system will not activate correctly, which increases the risk of a runaway trailer situation. It is vital for safety to ensure that the air lines are correctly connected before moving the trailer.