What does the Trailer Air Supply Control do? WHAT DOES THE TRAILER AIR SUPPLY DO? Think you’re ready for the CDL permit test? 🚛 See if you can answer this combination vehicle question: What does the Trailer Air Supply Control do? Drop your answer in the comments before you read on 👇 ✅ Correct answer: D — it operates the tractor protection valve and automatically applies the trailer’s emergency brakes if air pressure drops too low. This system is critical for safety between the tractor and trai