Commercial Driver's License

Answer: D - In some types of commercial vehicles that use air brakes, the vehicle will have a device that shows how much pressure is being applied to the brakes. This device is not the same as the supply pressure gauge that ALL air brake vehicles have in place. Also, all vehicles have a low air pressure warning system that alerts drivers when air pressure falls below 60 psi. The air governor controls the air compressor, allowing new air into the tanks when it is needed.