A tiny piece of debris -- a flake of rust, a gasket fragment, a piece of thread sealant -- wedged between the check valve disc and seat prevents it from sealing. The check valve is mechanically fine; it just needs cleaning. This is the most common and easiest sub-cause of check valve leaking.

What you'll see

Same as general check valve not sealing: continuous hiss from the unloader after the compressor stops, tank pressure dropping. The key difference: this often starts suddenly, right after maintenance work, right after a lot of rust was disturbed (e.g., after draining the tank for the first time in months), or after using thread sealant that got into the valve. The valve was working fine yesterday and today it leaks.
Before you assume this is the problem

If the check valve has been leaking for a long time and gradually gotten worse, it's more likely worn rather than having debris. See: Worn Check Valve Seat or Disc.

See all causes of air leaking from unloader when stopped →

How to diagnose

  1. Remove and inspect the check valve

    Drain the tank, remove the check valve, and disassemble it. Look carefully at the disc and seat for any foreign material: rust flakes, gasket pieces, thread tape fragments, or pipe scale. Even a tiny piece the size of a grain of sand can prevent the disc from seating. Clean everything thoroughly and look at the seat and disc under good light.

    Result: Debris found = clean and reassemble. No debris and surfaces look clean = disc or seat may be worn.

How to fix it

  1. Clean the valve thoroughly

    Wash all parts in solvent. Use a soft cloth to wipe the disc and seat. Don't use abrasives on the sealing surfaces -- you'll scratch them. Blow through the assembled valve to verify it seals in the correct direction. Reinstall with fresh thread sealant, being careful not to get sealant inside the valve (apply tape only to the first 2-3 threads, leaving the first thread bare).

  2. Prevent future contamination

    When applying thread sealant or tape near the check valve, don't over-apply. Excess tape can shred and get into the valve. When draining a tank that hasn't been drained in a long time, expect a lot of rust to come loose. After draining, let the compressor run and blow some air through before relying on the check valve -- the initial air carries the loosened debris.

Common mistakes

Over-applying thread tape is the biggest source of debris in check valves. One and a half wraps of PTFE tape is enough. Don't wrap it 10 times. Leave the first thread (closest to the end of the pipe) bare so the tape doesn't shred into the airstream. Also: don't use pipe dope (paste sealant) on the check valve itself -- it can get into the mechanism. Use it on the pipe connections upstream and downstream, not on the valve.

Parts & tools

Solvent and clean rags for cleaning. PTFE thread tape (applied carefully). Wrenches. Flashlight for inspection. No new parts needed if the valve itself is undamaged.

Review safety precautions before starting →

Safety

Drain all tank pressure before removing the check valve.

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...