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CDL · General

What is the tractor protection valve, and what does it do?

Correct answer

A valve that protects the tractor's air supply by closing automatically if trailer air pressure drops dangerously low (such as from a broken air line), preserving air for the tractor to stop

  1. A A valve that protects the tires
  2. B A valve that protects the tractor's air supply by closing automatically if trailer air pressure drops dangerously low (such as from a broken air line), preserving air for the tractor to stop
  3. C A device for the engine
  4. D A fuel valve

Why this is the answer

The TRACTOR PROTECTION VALVE protects the tractor's air brake supply in case of a major air leak in the trailer (such as a broken trailer air line) or if the trailer breaks away. CDL combination. It closes automatically (or the driver can close it via the trailer air supply valve) when air pressure drops to a low level (around 20-45 psi), SEALING OFF the tractor's air system so the tractor retains enough air to STOP SAFELY — preventing total air loss across both vehicles. When it closes, the trailer's spring brakes also apply (loss of air). This is a key fail-safe: a trailer air failure won't drain the tractor's air and leave it with no brakes. Knowing the tractor protection valve's function (seals off the tractor's air if trailer air is lost) is commonly tested.
Source: FMCSA CDL Manual — Combination, Tractor Protection Valve

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On a combination vehicle, what is the purpose of the trailer air supply valve (in the cab)?

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