Air Brakes
Air brakes are the highest-yield system on the exam. Questions cover components, the low-pressure warning, spring brakes, and the three-part leak and function test.
How the system works
A compressor charges the tanks through a governor. A dual-circuit system protects the front and rear circuits separately. The low-pressure warning must activate by 60 psi. Spring brakes apply when pressure falls to 20–45 psi. The tractor protection valve protects the trailer air supply in a combination vehicle.
The tests
With the system fully charged and the engine off, release the brakes and watch for one minute: loss should not exceed 3 psi on a single unit or 4 psi on a combination. Then apply the brakes fully and hold for one minute: loss should not exceed 4 psi. Pump down to check the warning at 60 psi and the spring brakes at 20–45 psi. Buildup should reach 85–100 psi within 45 seconds.
Worked example
A combination vehicle shows 120 psi. Engine off, brakes released, one minute later the gauge reads 115 psi - a 5 psi loss, over the 4 psi limit, so it fails. The driver should not just note it; an air leak can worsen and cause a brake to drag or fail. The vehicle stays out of service until repaired.
Traps
Running the leak test with the engine on; applying the 3 psi single-unit limit to a combination; fanning the brakes on a downgrade; and using the trailer hand valve for parking.