Air Brake System Operation and Inspection
What this domain tests
The air brake portion is the most technical part of the Class 1 exam. It tests components, the pressures that govern them, and the tests you must be able to perform and explain.
The system in brief
An engine-driven compressor charges the reservoirs through an air dryer. The governor cuts the compressor out at about 830-860 kPa (120-125 psi) and back in near 690 kPa (100 psi). Air passes from the wet tank to the primary and secondary tanks through one-way check valves; a safety valve protects the system near 1,035 kPa (150 psi). The foot valve signals the relay valve, which applies the trailer brakes. Spring parking brakes apply mechanically when air pressure is lost.
The tests you must know
- Build-up: at fast idle, pressure should climb from about 585 to 690 kPa (85-100 psi) in under 45 seconds.
- Leakage, brakes released, engine off: loss should not exceed about 14 kPa (2 psi) per minute on a single unit or 20 kPa (3 psi) on a combination.
- Leakage, brakes applied: about 28 kPa (4 psi) per minute single, 41 kPa (6 psi) combination. Hold the pedal firmly; do not pump.
- Low-pressure warning: buzzer and light must come on before pressure falls below 415 kPa (60 psi).
- Spring brake test: with the system charged, pull the parking valve and try to move; the brakes must hold. Most units also apply the spring brakes automatically between about 140 and 310 kPa (20-45 psi).
- Slack adjusters: with a full application, pushrod travel must stay within the limit for the chamber size. Have a mechanic adjust any automatic slack adjuster out of range; do not adjust it yourself to hide a problem.
Worked example
You run the released test on a tractor-trailer: pressure falls from 800 to 780 kPa in one minute, a 20 kPa loss. That is exactly at the combination limit, so you re-test after inspecting hoses and fittings, because a loss sitting at the limit will grow.
Traps that cost marks
Running the leak test with the engine on; forgetting to drain the tanks, which is critical in Quebec winters; confusing the tractor protection valve with the relay valve; and pumping the pedal during a leak test.