Not sure about big rigs, but on school busses the air is pressurized inside a container and used to keep giant heavy springs apart, when the pressure is released when the brakes are applied, the springs clamp down on the brake rotors slowing or stopping the bus. Air pressure comes from an air compressor run off the engine. Busses typically have between 70 and 100 psi in their pressure tanks, and most will not let you go into gear before that pressure is reached.
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