Air Brakes: Components, Pressures, and Tests
The system in one pass
A compressor (engine-driven) charges the tanks; the governor cuts the compressor out near 125 psi and back in near 100 psi. A safety valve protects the tanks from overpressure, and drain valves remove water and oil. The dual air system splits the circuit so a single failure still leaves braking on part of the vehicle. Service brakes are applied by air; spring brakes apply automatically when pressure drops too low, which is why you never rely on air alone for parking.
Numbers the examiner expects
- Low-pressure warning (light and buzzer) must activate by 60 psi.
- Spring brakes begin to apply somewhere between 20 and 45 psi.
- The gauge should build from about 85 to 100 psi within 45 seconds at fast idle.
- Slack adjusters should not move more than about 1 inch where the pushrod attaches, and the rod should not angle more than 45 degrees from the chamber.
The three tests
- Leakage test - fully charged, engine off, brakes applied; loss should not exceed about 3 psi per minute (4 psi for a combination).
- Low-pressure warning test - pump down and confirm light/buzzer by 60 psi and spring brake application by 45 psi.
- Applied/parking brake test - in a low gear, tug gently against the parking brake to confirm it holds.
Traps
Questions punish fanning the brakes on a downgrade, riding them, and skipping the drain of air tanks. Know that a leak in one circuit of a dual system should still leave service brakes on at least one axle pair.
Rule of thumb
If the pressure will not build or will not hold, do not move the vehicle - a slow leak is a failed test, not a nuisance.