Air Brake System Components and Functions
The Air Brake System Components and Functions domain tests whether you can name each part, state its job, and explain how the parts work together. Expect questions on the compressor, governor, reservoirs, valves, brake chambers, slack adjusters, and warning devices.
Air supply and storage
- Compressor: engine-driven pump that builds and maintains air pressure. It does not store air.
- Governor: senses reservoir pressure and controls compressor loading. It lets pressure build to cut-out (commonly about 125 psi) and restarts the compressor at cut-in (often near 100 psi).
- Reservoirs: store air. A supply tank feeds the system; a wet tank collects moisture and oil; a dry tank is protected by a check valve.
- Check valves: allow one-way flow so air cannot back up into a lower-pressure circuit.
- Safety valve: protects against overpressure, typically relieving near 150 psi.
- Drain cocks: remove moisture and oil from tanks.
Control and application
- Foot valve (treadle): driver-controlled service brake application.
- Brake chambers: convert air pressure into pushrod force. A spring brake chamber also contains a powerful parking spring.
- Slack adjusters: transfer pushrod motion to the foundation brake; poor adjustment wastes pushrod stroke.
- Foundation brakes: shoes and drums or discs that create friction.
- Low-pressure warning: must warn before pressure falls below 60 psi.
Worked example: After startup the gauge climbs to 125 psi and stops. That is normal governor cut-out. If it keeps climbing past 130, suspect a stuck governor or unloader; the safety valve is a backup, not a normal control. If the gauge falls below 60 with the engine off, the warning should sound-do not drive.
Traps: saying the compressor stores air; confusing the service and spring sections of a chamber; treating the safety valve as an operating control.