What happens when the driver crosses the air lines while hooking up to an old trailer?

When a driver crosses the air lines while hooking up to an old trailer, the situation can be quite concerning, particularly if the trailer lacks spring brakes. In this scenario, the brakes system operates differently than intended. The critical point is that if the trailer does not possess spring brakes, the driver may proceed to drive away; however, the trailer brakes will be inoperative. This lack of functioning trailer brakes can lead to severe safety problems, as the trailer could become uncontrollable when not properly braked, especially in emergency situations or while going downhill.

Understanding that the braking mechanism relies on air pressure, if the lines are crossed, the air pressure won't activate the trailer's braking system correctly. Since older trailers might not have modern braking systems such as spring brakes, the driver would be able to drive the truck without trailer brake functionality, creating an unsafe driving condition.