If the spring brakes do not release after connecting to a trailer, what is a probable cause?

When the spring brakes do not release after connecting to a trailer, one of the most probable causes is that the supply and service line are crossed. In a typical tractor-trailer braking system, the supply line delivers air to the trailer's brakes, while the service line controls the application of the brakes. If these lines are incorrectly connected, the trailer may not receive the appropriate air pressure needed to release the spring brakes. This misconfiguration prevents the brake system from functioning correctly, leading to potential safety hazards when attempting to drive.

Understanding the function of the air brake system and the importance of proper connections is crucial for anyone operating a tractor-trailer. This highlights the necessity of pre-trip inspections to ensure everything is set up correctly prior to movement.