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CDL · General

What is the function of the trailer hand valve (trolley valve) and why should it NOT be used for parking?

Correct answer

It applies only the trailer brakes and is used for testing; it should NOT be used for parking because it works on air pressure — if air leaks off, the trailer brakes release and the trailer could roll away

  1. A It applies only the tractor brakes; it's fine for parking
  2. B It applies only the trailer brakes and is used for testing; it should NOT be used for parking because it works on air pressure — if air leaks off, the trailer brakes release and the trailer could roll away
  3. C It controls the engine; never touch it
  4. D It's the emergency brake and should always be used for parking

Why this is the answer

TRAILER HAND VALVE (TROLLEY/TROLLEY VALVE): A hand-operated control that applies ONLY the trailer service brakes. USES: Primarily for TESTING the trailer brakes during pre-trip; some drivers misuse it, but it is NOT recommended for normal braking (can cause trailer skid/jackknife if used alone) or for parking; DO NOT USE FOR PARKING: The hand valve applies the trailer SERVICE brakes using air pressure; if the air bleeds off over time (normal slow leakage), the service brakes RELEASE and the trailer could roll away — there is nothing holding it; FOR PARKING: Use the PARKING BRAKES (spring brakes) — these are mechanically held by spring force and remain applied even if air pressure is lost (fail-safe); set the trailer and tractor parking brakes (push in the brake knobs); chock wheels on a grade; the distinction between air-applied service brakes (hand valve — temporary) and spring-applied parking brakes (mechanical — holds without air) is critical; never rely on the hand valve to hold a parked vehicle.
Source: FMCSA CDL Manual — Trailer Hand Valve

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