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CDL · General

Why is it important not to use the trailer hand valve (trolley valve) for normal braking?

Correct answer

Using the trailer hand valve alone can cause the trailer to skid or jackknife (it applies only the trailer brakes); normal braking should use the foot brake, which applies all brakes together

  1. A It works too well
  2. B Using the trailer hand valve alone can cause the trailer to skid or jackknife (it applies only the trailer brakes); normal braking should use the foot brake, which applies all brakes together
  3. C It applies the tractor brakes only
  4. D There is no reason

Why this is the answer

The TRAILER HAND VALVE (trolley valve) applies ONLY the trailer brakes. Using it alone for normal braking is NOT recommended because applying only the trailer brakes can cause the trailer to SKID or JACKKNIFE (especially when the trailer is empty/lightly loaded or on slippery roads). CDL combination. NORMAL braking should use the FOOT BRAKE (treadle valve), which applies the brakes on BOTH the tractor and trailer together in a balanced way. The hand valve is mainly for TESTING the trailer brakes during inspection. It should NEVER be used for PARKING (it works on air pressure that can bleed off — use the spring/parking brakes instead). Knowing not to use the hand valve for normal braking (skid/jackknife risk — use the foot brake) is commonly tested.
Source: FMCSA CDL Manual — Combination, Trailer Hand Valve Use

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