Air Brakes
Air Brakes tests the components, tests, and failure symptoms of a dual air system. You must know what the gauges should do, when the warning sounds, and what happens when pressure falls.
System basics
A compressor, governed by a cut-out around 120–125 psi and cut-in around 85–100 psi, charges wet and dry tanks. A dual system separates front and rear circuits so one failure still leaves some braking. The low-pressure warning must activate before pressure drops below 60 psi; spring brakes typically apply in the 20–45 psi range.
Required checks
- Build pressure from 85 to 100 psi within 45 seconds with the engine at operating rpm (dual system).
- With the engine off and brakes released, leakage should be no more than about 3 psi per minute; with brakes applied, about 4 psi per minute. Confirm exact limits in your manual.
- Test the parking brake, service brake, and emergency/spring brakes before driving.
Worked example
You start the engine and the gauge climbs from 85 to 100 psi in 40 seconds-passes the build test. You shut off the engine, release the brakes, and watch the gauge drop 2 psi in one minute-within the common 3 psi limit. With the brake pedal held down, it drops 5 psi-over the common 4 psi limit, so the system needs service before you move.
Traps
Confusing cut-in with cut-out; thinking the warning is optional; ignoring a slow pressure build; and assuming a dual system cannot fail completely.