Question The trailer hand valve: Answers Should always be used for parking. Should only be used for parking on a downhill. Should never be used for parking. Should only be used when transporting cargo. Explanation The trailer hand valve should not be used for parking. This could cause all of the air to leak out of the braking system, resulting in the brakes releasing. Instead, use the parking brake. Take a FREE Practice Test Now This questions appears in the following tests: Kansas Double , Florida Combination , Indiana Combination , Maryland Combination , Massachusetts Combination , Mississippi Combination , Nevada Combination , South Carolina Combination ,