Heavy-Vehicle Braking Systems and Air Brakes
The core idea
Air brakes work on pressure, and their failure modes are different from hydraulic brakes. Questions test how the system builds pressure, what warning signs mean, and what to do when pressure is lost.
Concepts to master
- Air supply: compressor, reservoirs, governor, and safety valve. Know how pressure builds and the normal operating range.
- Warning devices: low-pressure warning and the emergency spring brakes that apply when pressure falls too low.
- Brake types: service brakes, parking/spring brakes, and engine or retarder braking.
- Leak checks: a static leak test and a pressure build-up test; know that a slow leak is still a defect.
- Brake fade: overheating reduces braking; use lower gears and retarders on descents.
- ABS and EBS: electronic systems assist but do not replace safe speed and following distance.
Traps in questions
- Assuming low pressure means "drive to the workshop"; it means stop.
- Forgetting that spring brakes apply automatically at low pressure.
- Treating a slow leak as acceptable.
- Believing ABS shortens stopping distance on all surfaces.
Worked rule of thumb
If the low-pressure warning sounds or pressure falls below the safe range, stop safely and do not continue. The spring brakes will apply as pressure drops, so plan a safe stop before they do.
Key points
- Air brakes operate on pressure; loss of pressure applies spring brakes.
- Low-pressure warning means stop, not continue.
- Slow leaks are defects and must be reported.
- Use gears and retarders to prevent brake fade on descents.
- ABS assists control but does not replace safe speed.