Air Brakes: Components, Tests, and Failure Modes
Maryland's Air Brakes domain tests the components, tests, and failure modes of a vehicle air brake system. You must know what the gauges tell you and the order in which to perform the CDL air brake checks.
System basics
A CMV air brake system has three braking systems: service, parking, and emergency. The compressor builds pressure; the governor cuts out near 125 psi and cuts in near 100 psi. A dual-circuit system protects steering and drive axles separately. The low-pressure warning must activate before pressure falls below 60 psi. Spring brakes apply automatically between about 20 and 45 psi. Do not fan the brake pedal; use stab braking, applying firmly, releasing when wheels roll, and reapplying.
Tests and adjustment
With the engine off and brakes released, a single vehicle should lose no more than 3 psi in one minute; a combination, no more than 4 psi. With the brakes applied, the same limits apply. Check the warning buzzer and spring-brake application. Test the service brakes at low speed. Check slack adjusters and pushrod travel; excessive travel means the brakes need adjustment. ABS is a supplement, not a substitute: keep normal braking and steer.
Worked example
A combination starts at 120 psi. After one minute with the engine off and brakes released, the gauge reads 116 psi. The drop is 4 psi. The combination limit is 4 psi, so it is at the limit; record it and recheck. If it drops further, the vehicle fails the leak test.
Traps
Pumping the pedal instead of stab braking; using the parking brake as a service brake; forgetting to drain air tanks; testing only at low pressure; assuming ABS shortens stopping distance on all surfaces; and not knowing that spring brakes are the emergency system.