What is the trailer hand valve, also called the trolley valve or Johnson bar, intended to be used for?
Combination Vehicles • Free CDL Practice Question
Regulation
AAMVA CDL Manual, Section 6.2.1 "Trailer Hand Valve," p. 6-4
Why This Matters
The trailer hand valve looks like a convenient shortcut for controlling just the trailer, but the manual is explicit that its only legitimate use is a brake test. Using it any other way introduces real jackknife risk.