Review your state's department of motor vehicles web site for information on air brakes; many states have online descriptions of air brakes that may be viewed or downloaded for free. For example, the California Department of Motor Vehicles' Commercial Driver License Handbook offers a section on air brakes (free online) which describes the three brakes in the air brake system, requirements for air gauge and pressure warnings, along with minimum percentages of air reservoir pressure.
Purchase historical or introductory books on air brakes such as "Evolution of the Air-Brake" by Paul Synnestvedt, "The 2009-20014 World Outlook for New Motor Vehicle Air Brake Power Actuation Units" by the Icon Group, "Diseases of the Air Brake System" by Paul Synnestvedt, or "Introduction to the Study of Air Brakes," by Walter V. Turner. Books are available through online retailers such as Barnes and Noble, Book Closeouts and Amazon.com.
Check your local community college to see if a course in air brakes is offered. Topics may include air control values, air system schematics, servicing, preventive maintenance and test system components. If your nearest school doesn't offer such a course, check for courses on large vehicle repair or commercial truck maintenance, then contact the instructor to inquire whether air brakes are covered.
Test your knowledge of air brakes by taking an online air brake test. Many tests are offered free from Commercial Drivers' License training schools, but you're not required to sign up or enroll to take the test. If the commercial driving field interests you, ask school instructors how much time is spent on air brakes.