Large trucks use compressed air brake systems. It is the most preferred braking system in large trucks, buses, trailers and semi-trailers. An air brake system uses compressed air to transmit pressure from the driver's control to the service brake.
An air braking system couples multiple vehicle units giving them braking capability and allowing control of units brakes from the cab, which is impossible with hydraulic brake systems. Additionally, it allows incorporation of an emergency braking system that utilizes part of the service brake and parking brake systems. Typically, an air brake system is a combination of three braking systems: the service brake, the parking brake and the emergency brake systems.