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CDL · Antilock Brake Systems

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

Reviewed by Jordan Morley, Editorial Lead at 247SimpleTests. Written by the 247SimpleTests Editorial Team from the official source material cited on every question.

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 adjusts brake balance between tractor and trailer
  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 is only used during pre-trip inspection
  4. D It controls only the tractor's air brakes

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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About this topic: Antilock Brake Systems

ABS prevents wheel lockup during heavy braking, preserving steering and reducing stopping distance on slick surfaces. Drivers must brake firmly, not pump, on ABS-equipped vehicles. This is one of 6 Combination Vehicles questions on antilock brake systems, each with a full explanation on a single study page.

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