Air Brakes on Combination Vehicles Practice Test, Test 1 of 2
Question 1 of 12
What is the trailer hand valve, also called the trolley valve or Johnson bar, intended to be used for?
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Not quite.
Wrong. The manual says the hand valve should be used only to test the trailer brakes, not during normal driving, because of the danger of making the trailer skid.
The correct answer is B: Testing the trailer brakes only.
Correct. AAMVA Section 6.2.1: "The trailer hand valve... works the trailer brakes. The trailer hand valve should be used only to test the trailer brakes."
Wrong. The tractor protection valve is a separate component controlled by the trailer air supply control valve (Section 6.2.2), not by the trailer hand valve.
The correct answer is B: Testing the trailer brakes only.