Practice CDL knowledge test: Question 18 of 20

The trailer hand valve (also called the trolley valve or Johnson bar) works the trailer brakes. Don't use it in driving because of the danger of making the trailer skid. The trailer hand valve should only be used when:

A	There is ice and snow, to prevent skidding.
B	Testing the trailer brakes after hooking up to a trailer.
C	You will be parked for a short time.

A	INCORRECT The correct answer is: Testing the trailer brakes after hooking up to a trailer. Next question

B	CORRECT Testing the trailer brakes after hooking up to a trailer. Next question

C	INCORRECT The correct answer is: Testing the trailer brakes after hooking up to a trailer. Next question