Air and Hydraulic Suspension Controls
What the candidate must know
Understand the air supply path: compressor, governor, dryer, reservoir, height control valve(s), air springs, and exhaust. Know the governor cut-in/cut-out function, air dryer purge and desiccant condition, and how HCV linkage commands ride height. For hydraulic systems, know hydraulic leveling/assist components, accumulators, and pressure checks. Recognize that school buses may use air suspension on the rear only or all positions, and that brake air supply shares the compressor - a suspension leak can drop brake pressure.
Traps in questions
- Forgetting that a stuck HCV in the exhaust position will lower the bag even with a good air spring.
- Overlooking that air dryer desiccant or a failed purge valve sends moisture to valves and causes freeze-ups and HCV failure.
- Assuming a slow leak is only the bag - check lines, fittings, and the HCV itself with soap solution.
- Missing that governor cut-out too high or too low affects the whole air system, not just suspension.
Rule of thumb / worked example
For a bus that will not hold ride height: build air to governor cut-out, shut down, and watch the gauge. A drop with no audible leak points to an internal leak (HCV or bag); a hiss points to a line or fitting. Soap-test every joint and the HCV exhaust port. Confirm the governor cut-in/cut-out is within spec and the dryer purges before replacing parts. Always verify brake air pressure is unaffected.