When the compressor stops, air keeps hissing out of the unloader valve or pressure switch: usually a check valve problem, not the unloader.
What this problem usually means
When the compressor stops, the unloader valve opens to vent the discharge line so the compressor can restart easily. This is normal, you should hear a brief "psssht" of air.
But if air keeps hissing out continuously, the tank check valve is leaking backward. Air from the receiver tank is flowing back through the check valve, through the discharge line, and out the unloader. The unloader is doing its job, the check valve isn't.
Check these first
5–10 minute checks before diving deeper
- Listen to confirm the leak is from the unloader/pressure switch area
- Watch the tank pressure gauge: is pressure slowly dropping?
- Locate the check valve: usually between compressor head and tank
- Check if check valve is installed in correct direction
- Try tapping the check valve lightly: sometimes debris dislodges
- Is the leak constant or does it stop after tank empties?
- Check for any visible damage to check valve body
- When did this start? After service work on the check valve?
Common root causes
Why this happens in industrial reciprocating compressors
- Check valve not sealing The tank check valve has debris, a worn seat, or damaged disc. Air leaks back from tank to discharge line and out the unloader.
- Debris in check valve Pipe scale, rust, or debris preventing the check valve from seating. Remove, clean, and reinstall the valve.
- Check valve installed backward If recently serviced, verify the arrow on the valve body points toward the tank (direction of flow). Backwards installation won't seal.
- Worn check valve seat or disc Internal sealing surfaces are worn or corroded. Valve needs rebuild kit or replacement.
Don't replace the unloader valve or pressure switch first—they're rarely the problem when air leaks continuously after stopping. The issue is almost always the tank check valve.
Release tank pressure before removing the check valve. Compressed air in the tank will rush out when the valve is removed.
Still stuck?
If the checks above haven't pointed at the cause, post your symptoms in the Q&A. Real-world answers, no sales pitch.