Air Brakes
The system
Air brakes use compressed air instead of hydraulic fluid and include three braking systems: service, parking, and emergency. Understanding how air pressure builds and fails is central to the test.
Concepts to master
- Compressor and governor: The compressor builds pressure; the governor cuts it out around 125 psi and back in around 100 psi.
- Warning signals: A low-air warning must activate by 60 psi. The spring brakes should apply between 20 and 45 psi.
- Leak test: With the engine off and the system charged, pressure should not drop more than 3 psi in one minute for a single vehicle, or 4 psi for a combination.
- Alcohol evaporator prevents moisture from freezing in air lines.
- Drain the tanks daily to remove water and oil.
- Spring brakes apply automatically when air pressure is lost; they are held off by air pressure.
Traps
Examiners test whether you know that a low-air warning is not the same as the spring brakes applying. Another favorite: if the compressor fails, you have only the air already in the tanks, so stop immediately. Also remember that on a long downgrade you should use a low gear and stab braking, not ride the brakes.
Worked example
During a leak test on a combination, pressure drops from 120 to 115 psi in one minute - a 5 psi drop. The limit is 4 psi, so the vehicle fails the test and must be repaired before driving.
Rule of thumb
Build pressure to the governor cut-out, shut the engine off, and watch the gauge - if it keeps falling, do not drive.