Air brake systems and heavy vehicle braking
Air brake systems and heavy vehicle braking
This domain tests how the air system works, which tests prove it will stop, and how to brake a loaded combination on a long descent without destroying the brakes.
How the system works
- An engine-driven compressor charges a wet tank and then dual service circuits through a foot valve. A protection valve isolates a failed circuit so partial braking remains.
- Governor cut-out sits near 810–830 kPa and cut-in near 690 kPa.
- Spring brakes are held off by air; losing pressure applies them. Loss of air is a brake application, not a release.
- Brake chambers (type 20, 24, 30) push slack adjusters, which move the shoes. Excess pushrod stroke means the brakes are out of adjustment. ABS prevents wheel lock but does not shorten stopping distance on sand or gravel.
Tests before moving
- Build-up test from zero to cut-out, governor check, low-pressure warning by about 420 kPa.
- Static leak test: engine off, full pressure, brakes released then applied; typical allowance is around 20 kPa per minute for a combination.
- Tug test after coupling to prove the trailer brakes and coupling hold.
Braking technique on descents
- Select a low gear before the grade and let engine braking carry the load.
- Use snub braking: a firm application to shed 10–15 km/h, then release. Light continuous dragging overheats drums and causes fade.
Worked example
A 40 t combination on a 6% grade covers 2 km with continuous light pedal pressure. The pedal then travels further and braking fades. Reasoning: the drums are overheated. The response is to downshift, use engine or retarder braking, apply firm snubs, and if fade continues take an escape route or stop to cool - never press harder.
Traps
- Fanning the pedal, or using spring brakes as a service brake on a grade.
- Accepting a slow build-up because the gauge eventually climbs.