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What do the two gladhand colors on a tractor-trailer connection stand for?
- A.Red is the service line and blue is the emergency line
- B.Red is the emergency (supply) line and blue is the service line
- C.Both colors carry the same air for decoration
- D.Red is for the left trailer and blue is for the right trailer
Correct answer
B. Red is the emergency (supply) line and blue is the service line
Why
Color coding keeps the two circuits straight: the red emergency line charges the trailer tanks and controls its spring brakes, while the blue service line carries the signal that applies the service brakes. Crossing them produces a trailer that will not brake or release correctly.
Manual topic: Combination Vehicles - Air Lines and Gladhands · From the Combination Vehicles practice bank (CV-236).