Air Supply and Service System Components
Supply side vs. service side
The supply system makes and stores air; the service system delivers it on demand. The engine-driven compressor pumps through a discharge line to the air dryer, then into the wet tank, and through a one-way check valve into the dry tank(s) that feed the foot valve. The governor senses reservoir pressure and unloads the compressor at cut-out (typically 120–125 psi) and lets it reload at cut-in (typically 85–100 psi).
Components to be able to name
- Air dryer: desiccant cartridge, purge valve and purge cycle, discharge check valve, heater circuit; removes moisture and oil aerosol before the wet tank.
- Check valves: one-way and double check valves stop backflow and select the higher of two pressure sources.
- Pressure protection valve (PPV): opens around 65 psi and isolates accessories (horns, suspension, seats) so the brake system keeps priority.
- Safety valve: on the wet or dry tank, usually set near 150 psi; it is a backup for a failed governor, not a regulator.
- Low-pressure warning: continuous audible and visual warning must activate before pressure falls to 60 psi.
- Foot valve, relay valve, quick-release valve: dual-circuit treadle valve; relay valves speed rear apply and release.
Traps and rules of thumb
Questions frequently swap cut-in and cut-out, or suggest the safety valve sets system pressure. Know the leak standards: no more than 2 psi/min with brakes released and 3 psi/min with brakes applied (confirm the published spec for the vehicle combination). Build time: 85 to 100 psi in 45 seconds at fast idle.
Rule of thumb: governor cut-in sits roughly 20–25 psi below cut-out, so a 125 psi cut-out should reload near 100 psi.