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CDL · General

In a combination vehicle, what is the purpose of the trailer air supply valve (sometimes called the tractor protection valve control)?

Correct answer

It supplies air to the trailer brake system; closing it (pushing it in) cuts off air to the trailer and applies the trailer spring brakes — used to protect the tractor's air supply if a trailer line breaks

  1. A It controls only the tractor's air brakes
  2. B It supplies air to the trailer brake system; closing it (pushing it in) cuts off air to the trailer and applies the trailer spring brakes — used to protect the tractor's air supply if a trailer line breaks
  3. C It adjusts brake balance between tractor and trailer
  4. D It is only used during pre-trip inspection

Why this is the answer

The TRAILER AIR SUPPLY VALVE (red octagonal knob) opens the tractor protection valve, allowing air to flow to the trailer. If a trailer air line breaks, air pressure will drop rapidly — the tractor protection valve automatically closes when pressure drops below approximately 20-45 psi, sealing the tractor's air system and applying the trailer spring brakes. Manually closing this valve does the same thing on purpose. Know its location in the cab — in an emergency, closing it stops a runaway trailer.
Source: FMCSA CDL Manual Chapter 6, Combination Vehicles, Tractor Protection Valve

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On a combination vehicle, what happens to the trailer if the air line between the tractor and trailer breaks while driving?

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