On tractor-trailer combinations, service brake signals from what are sent to the trailer?

Answer: C — Both systems (primary and secondary)

The service brake signals from both the primary and secondary systems are sent to the trailer in a tractor-trailer combination to ensure effective braking performance and safety. The primary braking system is responsible for normal braking operations, allowing the vehicle to decelerate smoothly under typical driving conditions. The secondary system serves as a backup, enhancing safety by ensuring that if the primary system fails, the secondary system can still provide adequate braking. This dual-signal approach is crucial for maintaining control of the entire vehicle combination, particularly when operating under varying loads and road conditions. Sending signals from both systems increases the reliability of the braking function, allowing the trailer to respond promptly to the tractor's braking actions. Such redundancy is vital in heavy vehicles to prevent accidents caused by brake failure, making it critical for operators to understand how these systems work together.