When should the trailer hand valve be used? Question When should the trailer hand valve be used? Answers Only when the trailer is carrying a load Only to test the trailer’s brakes Only in inclement weather when the trailer may skid Explanation The trailer hand valve, also called the trolley valve or Johnson bar, is used to work the trailer brakes. It should be used only to test the brakes. Using it while a vehicle is in motion could cause a skid. This questions appears in the following tests: Alaska Combination Arkansas Combination Louisiana Combination New Mexico Combination South Dakota Double South Dakota Combination Utah Combination