An air brake system on a truck tractor is actually three separate braking systems sharing components. Which three systems make up a vehicle's air brake system?

49 CFR 383.111(b)(1) (general air brake system nomenclature); AAMVA CDL Manual, Section 5 introduction, p. 5-1

Understanding that air brakes are three overlapping systems, not one, explains why losing air pressure does not simply disable braking. It is the conceptual foundation for every other air brake topic, including why spring brakes exist as a mechanical backup.