Question Vehicles with dual air brake systems: Answers Maintain full-capacity air pressure in the braking systems even when the vehicle is not running. Maintain some air pressure in the braking systems when the vehicle is not running. Must be run to build enough air pressure to brakes to operate properly. Do not need time to build air pressure before a trip. Explanation Before driving a vehicle with a dual air brake system, you must allow the air compressor time to build up at least 100 psi of air pressure in both the primary and secondary systems. Take a FREE Practice Test Now This questions appears in the following tests: Colorado Knowledge Test Class B , Kansas Knowledge Test Class B , Pennsylvania Knowledge Test Class B , Georgia Air Brakes , South Dakota Air Brakes , Wisconsin Air Brakes ,