Air Brakes: Pressure, Tests, and Brake Fade
Colorado's Air Brakes test is 25 questions with an 80% passing score and is required for any vehicle with air brakes. It tests how the system builds and loses pressure, the tests you must perform, and how to brake without overheating.
How the system works
- The compressor builds pressure; the governor cuts the compressor out near 125 psi and back in near 100 psi.
- A warning light and buzzer must activate before pressure drops below 60 psi. If it does, stop - you may not have enough pressure to stop safely.
- Spring brakes apply automatically at roughly 20–45 psi. They are the parking brakes and the emergency backup.
- The system has three braking systems: service (foot valve), parking (spring), and emergency.
The tests
- Static leak test: engine off, system fully charged, parking brake released. Watch the gauge for one minute - loss should be less than 2 psi in a single vehicle, less than 3 psi in a combination.
- Then hold the foot brake fully applied for one minute: loss should be less than 3 psi.
- Applied leak test: engine running, pressure at 90 psi, fully apply the brake, shut off the engine, wait one minute; loss should be no more than 3 psi.
- Check slack adjusters by hand: if one moves more than about 1 inch, the brakes may be out of adjustment.
- Drain air tanks daily to remove moisture.
Worked example
A combination's gauge reads 120 psi. Engine off, parking brake released, after one minute it reads 116 psi - a 4 psi loss. The combination limit is less than 3 psi, so this fails: a leak must be found before driving. A 2 psi loss would pass for a combination but would fail the single-vehicle standard, which requires less than 2 psi.
Using air brakes
- On long downgrades, select a lower gear before the descent and use snub braking - apply firmly, then release - rather than riding the brakes.
- Brake fade means the brakes are overheated; stopping distance grows. Pull over and let them cool.
- Never use the parking brake to make a normal stop.
Traps
- Confusing cut-in (about 100 psi) with cut-out (about 125 psi).
- Thinking spring brakes apply at 60 psi - 60 psi is the warning threshold; the springs apply at 20–45 psi.
- Running the leak test with the engine running or the parking brake applied.
- Forgetting that a 2 psi drop passes a combination but fails a single vehicle.