The check valve between the pump and tank prevents air from flowing backward. When it doesn't seal, air from the tank leaks back through the pump and out the unloader valve. You hear a continuous hiss from the unloader, and tank pressure drops steadily. Most common cause of this symptom.

What you'll see

After the compressor stops, you hear a continuous hissing from the pressure switch / unloader valve area. The tank pressure gauge slowly drops. The longer the compressor is off, the more pressure you lose. In severe cases, the tank empties completely between cycles. The unloader did its normal blow-down when the compressor stopped, but then air continues to hiss because the check valve is letting tank air leak backward through the pump head and out the open unloader.
Before you assume this is the problem

If the air is leaking from somewhere other than the unloader (a fitting, the safety valve, or a drain), it's not a check valve issue -- it's whatever's leaking. If the hissing only lasts a second or two and then stops, that's the normal unloader blow-down, not a check valve problem.

See all causes of air leaking from unloader when stopped →

How to diagnose

  1. Listen for continuous hissing after blow-down

    When the compressor stops, the unloader should blow down for 1-2 seconds and then go silent. If the hissing continues after the initial blow-down, air is leaking backward through the check valve. The check valve is usually a brass or steel fitting where the discharge pipe enters the tank.

    Result: Silent after blow-down = check valve sealing. Continuous hiss = check valve leaking.

  2. Watch the tank pressure gauge

    With the compressor off, watch the tank gauge. If pressure drops steadily (you can see the needle moving over a few minutes), and the hissing is coming from the unloader, the air path is: tank -> check valve (backward) -> pump head -> unloader (out). The check valve is the failed component in this chain.

    Result: Pressure holds = check valve OK. Pressure drops with hissing = check valve leaking.

How to fix it

  1. Clean or replace the check valve

    Drain the tank. Remove the check valve (usually threaded into the tank). Disassemble it and clean the disc, spring, and seat. Look for debris, carbon buildup, corrosion, or a cracked/warped disc. If the seat is scored or pitted, cleaning won't help -- replace the valve. Apply thread sealant to the threads when reinstalling.

  2. Test after repair

    Run the compressor to full pressure, let it stop, and listen. The hissing should stop within a few seconds (the normal blow-down). Watch the tank gauge for 10-15 minutes -- pressure should hold steady. If it still drops, the replacement check valve may also be defective (rare but possible) or there's another leak in the system.

Common mistakes

Installing the check valve backward -- the arrow on the body must point toward the tank. Also: the blow-through test (blowing air through the valve by mouth) only proves the valve opens -- it doesn't prove it seals. A valve can pass the blow-through test and still leak under the higher pressure of a full tank. Replace if in doubt -- they're cheap. And don't over-tighten into the tank -- especially on older tanks where the threads may be corroded or thin.

Parts & tools

Replacement check valve (match thread size and type). Thread sealant tape. Wrenches. Clean rags and solvent for cleaning.

Review safety precautions before starting →

Safety

Drain all tank pressure before removing the check valve. The check valve is under full tank pressure. Use eye protection.

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...