Air Brake System Knowledge
How the system works
A basic air brake system has a compressor, an air dryer, reservoirs (wet, primary, and secondary tanks), a foot valve, brake chambers, slack adjusters, and push rods. The compressor builds pressure, the governor cuts it out around 120-125 psi and cuts it back in around 100 psi, and the safety valve protects the system from overpressure. Understand that air brakes are a spring-applied, air-released system: strong springs apply the parking brakes, and air pressure releases them. That is why a loss of air pressure causes the brakes to apply, which is a fail-safe design.
The tests you must know
- Leak-down (static) test: with the engine off and the system fully charged, release the parking brake and watch the gauge. A loss of more than about 3 psi in one minute for a single vehicle (or about 4 psi for a combination) indicates a leak.
- Applied (leakage) test: hold a full brake application and watch for excessive pressure loss.
- Low-pressure warning: the warning buzzer and light should activate before pressure drops below about 60 psi.
- Spring brake (emergency) test: with pressure below about 45 psi, the spring brakes should fully apply.
- Compressor build-up: the system should build from about 85 to 100 psi within a reasonable time, commonly about 45 seconds.
The traps
- Fanning the brakes. Pumping the pedal on a long downgrade uses up air and can leave you without brakes. Use a steady application and a low gear instead.
- Not knowing the cut-in and cut-out pressures. Questions test whether you understand normal gauge behaviour.
- Ignoring the air dryer. Moisture in the system causes freeze-ups and corrosion.
Rule of thumb
If the low-pressure warning sounds while driving, stop safely and find the cause. Never drive a vehicle that will not build or hold air pressure.