Bus-Specific Vehicle Systems: Air Brakes, ABS, and Stopping Distance
What this domain tests
This domain tests air-brake operation and checks, ABS awareness, and the factors that increase stopping distance for a passenger bus.
Air brakes
Know the compressor, governor, gauges, and spring brakes. The low-pressure warning must activate by 60 psi; below that, stop safely. Spring brakes apply when pressure falls too low, typically in the 20 to 45 psi range. Perform the air-brake check: build pressure, check leakage, and confirm the warning system.
ABS and stopping distance
An ABS lamp means the system has a fault; normal brakes still work, but skid control may not. Do not pump ABS brakes; apply firm, steady pressure. Total stopping distance includes perception, reaction, brake lag, and actual braking. Air-brake lag adds distance. Speed, weight, road surface, brake condition, and load all increase stopping distance.
Worked example
During a pre-trip check, the low-pressure warning sounds at about 60 psi while the gauge is falling. You should stop the check and place the bus out of service. Reasoning: the warning system is working, but the system is losing pressure; driving with an air leak is unsafe.
Traps
- Fanning the brakes down a long grade instead of using a lower gear.
- Ignoring an ABS warning lamp.
- Believing ABS always shortens stopping distance.
- Forgetting brake lag when estimating stopping distance.