Component
Check valves hold receiver pressure; unloader valves bleed head pressure on shutdown so the compressor restarts unloaded.
Operating principle
A check valve lets air flow compressor→receiver only, holding tank pressure during off cycles. An unloader valve opens on shutdown to bleed compressed air from the compressor head/pipes to atmosphere so the next start is unloaded (low torque).
Components
- Check valve (disc/poppet/spring)
- Unloader valve (solenoid or pressure-switch-actuated)
- Unloader line to atmosphere
- Pressure-switch interlock
Common issues
- Failed check → tank bleeds back, hard restart, compressor runs on restart
- Stuck unloader → motor stalls/loads on restart, blown breaker
- Leaking unloader → continuous air loss, short cycles
- Unloader line clogged → head stays loaded
Maintenance
- Inspect check-valve seat for debris at PM
- Test unloader cycles on each shutdown
- Clean unloader orifice/line
- Replace diaphragm/poppet kits per OEM
Field tips
- Hard starting / blown breaker on restart = unloader failing
- Short cycling or hissing after shutdown = unloader leak
- A brief bleed-down hiss on shutdown is normal; a continuous hiss is not