Air Brakes for the Oregon CDL School Bus Test
What the Air Brakes domain asks
The Air Brakes domain checks your understanding of the dual air system, warning devices, spring brakes, and the tests you must perform. A school bus with air brakes will not pass a pre-trip if the system leaks or warns too late.
Core rules
- The compressor, governed by the governor, charges the wet tank and then the primary and secondary reservoirs. The foot valve applies service brakes; spring brakes apply when air pressure drops.
- Low air pressure warning must activate by 60 psi. The spring brakes should apply between 20 and 45 psi.
- Know the three air brake tests: compressor build-up, static leak, and applied leak. With a fully charged system and engine off, a single vehicle should lose less than 3 psi in one minute; with the brakes applied, less than 4 psi per minute.
- Check slack adjusters and pushrod travel during the pre-trip. Do not adjust slack adjusters unless you are trained.
- Do not fan the brakes on long downgrades. Use a low gear and controlled braking; brake fade means the brakes are overheating.
Worked example
After draining pressure to 85 psi, you run the engine and time the build-up. The gauge reaches 100 psi in 50 seconds. That fails the 45-second standard for a dual air system, so the bus should be placed out of service until the compressor or governor is checked.
Traps
- Confusing the low-pressure warning (60 psi) with spring-brake application (20–45 psi).
- Testing with the engine running and calling it a static leak test.
- Relying on engine braking alone on a steep grade.
- Ignoring a slack adjuster that moves more than the allowed travel.