After thousands of open-close cycles, the check valve disc and seat wear against each other. The seat becomes pitted or eroded, the disc becomes scored or warped. Cleaning doesn't help because the surfaces themselves are no longer flat. Time for a new valve.

What you'll see

Gradual worsening of the check valve leak over time. It started as a barely noticeable hiss and slowly got worse. Cleaning the valve helps temporarily but the leak returns quickly. The valve has been in service for years. When you remove and inspect it, the disc has a groove worn into it, or the seat has pitting and erosion that you can feel with your fingertip.
Before you assume this is the problem

If the leak started suddenly rather than gradually worsening, it's more likely debris than wear. If the valve is relatively new, check for debris or installation issues first. See: Debris in Check Valve, Check Valve Installed Backward.

See all causes of air leaking from unloader when stopped →

How to diagnose

  1. Inspect disc and seat surfaces

    Remove the valve and look at the sealing surfaces under good light. Run your fingernail across the seat -- if you feel grooves, pitting, or roughness, the seat is worn. Check the disc for warping (place on a flat surface) and for a worn groove where it contacts the seat. Hold the disc against the seat and look for light between them -- any gap means it can't seal.

    Result: Smooth, flat surfaces = not worn. Pitted, grooved, or warped = worn out, replace.

How to fix it

  1. Replace the check valve

    Once the seat or disc is worn, replacement is the only reliable fix. Match the thread size and type (NPT, BSP, etc.) and the pipe diameter. Apply thread sealant and install with the flow arrow toward the tank. They're typically $10-$30 and available at most hardware stores or compressor parts suppliers.

Common mistakes

Trying to lap or resurface a worn check valve seat. On a small compressor check valve, this is rarely worth the effort -- a new valve costs less than the time spent trying to fix the old one. Also: don't replace a brass check valve with a plastic one to save money. Plastic valves have shorter lifespans and can't handle the heat of the discharge air as well. Use brass or steel matching the original quality.

Parts & tools

Replacement check valve (match thread size and direction). Thread sealant tape. Wrenches.

Review safety precautions before starting →

Safety

Drain all tank pressure before removal.

This issue can also cause

  • Compressor Won't Start Motor hums, trips the breaker, or nothing happens: usually unloader valve, check valve, or electrical problems...