A post-repair mistake: the check valve was reinstalled or replaced with the flow direction reversed. A backward check valve blocks air from reaching the tank (so the compressor can't fill the tank) or allows unrestricted backflow (tank drains through the pump). Either way, the system doesn't work.

What you'll see

Two possible scenarios: (1) The compressor runs but the tank never fills -- the backward check valve blocks the air from entering the tank. The safety relief valve on the pump head may pop because pressure builds behind the blocked valve. (2) The tank fills but drains rapidly when the compressor stops -- the backward valve allows free backflow. Both scenarios start immediately after the check valve was replaced or reinstalled. The compressor worked fine before the repair.
Before you assume this is the problem

If the check valve hasn't been recently touched, it's not installed backward -- it's either worn or has debris. See: Check Valve Not Sealing, Debris in Check Valve.

See all causes of air leaking from unloader when stopped →

How to diagnose

  1. Check the flow direction arrow

    Most check valves have an arrow cast or stamped into the body showing the correct flow direction. The arrow should point toward the tank (from pump to tank). If it points away from the tank, the valve is backward. If there's no arrow, the valve may be symmetrical or the marking has worn off -- in that case, blow through it from each end. Air should pass toward the tank direction and block in the other direction.

    Result: Arrow toward tank = correct. Arrow away from tank = backward, reverse it.

How to fix it

  1. Reverse the check valve

    Drain the tank. Unscrew the check valve, turn it around, and reinstall with the arrow pointing toward the tank. Apply fresh thread sealant. Tighten snugly. Run the compressor and verify: it should fill the tank normally, and after stopping, the tank should hold pressure without hissing from the unloader.

Common mistakes

Not all check valves have arrows. If you can't determine the direction, test it before installing: blow through it from each end. You should be able to blow through in one direction and not the other. Mark the flow-through direction with a marker and install accordingly. Also: some check valves look identical from both ends -- take a photo before removal so you know which way it was installed.

Parts & tools

Wrenches. Thread sealant tape. Marker for direction marking. Phone camera for documenting orientation.

Review safety precautions before starting →

Safety

If the safety relief valve is popping because of a backward check valve, shut down immediately. Drain the tank before reversing the valve.

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