What must air brake equipped vehicles have?

A An air pressure gauge to show pressure available for braking

explanation

All vehicles with air brakes have a pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system. (Or a single gauge with two needles.) Dual systems will be discussed later. These gauges tell you how much pressure is in the air tanks.
[Commercial Driver’s License Manual]