Air compressor check valve: symptoms, testing and replacement
What the check valve does
When the motor stops, the pressure switch's unloader vents the short line between the pump and the tank, so the motor can restart without fighting pressure. The check valve keeps the tank's air from following it out. If the valve sticks open or its seal wears, tank air leaks back through the unloader.
Symptoms of a bad check valve
- Continuous hissing from the unloader/pressure switch after the compressor shuts off.
- Tank pressure drops while the compressor sits, with no leaks at fittings.
- Motor hums, labors or trips the breaker when trying to restart under pressure.
- Air escaping from the pump intake (a different fault, a pump valve, but often confused with this one).
How to test it
The steps below are summarized in the how-to section. Always follow your compressor's manual first.
Replacing it
Check valves are cheap brass parts. Note the thread size and the direction of flow (usually an arrow), drain the tank fully, then disconnect the discharge tube and unloader tube and unscrew the valve. Apply thread sealant as the replacement's instructions say, and leak-test with soapy water afterward. See replacement parts.
Safety first: switch off and unplug the compressor, then drain the tank completely before loosening any fitting, valve or line. Compressed air stored in a tank can turn a fitting into a projectile. Wear eye protection. Leave tank damage and electrical work on motors to a qualified technician.
Types of check valve
- In-tank check valves screw directly into the tank, with a fitting for the discharge tube and often a small port for the unloader line.
- Inline check valves sit in the discharge line on some larger compressors.
- Most use a spring-loaded disc or poppet. Debris on the seat or a weak spring is the usual cause of leaks.
Cleaning vs replacing
With the tank fully drained, some check valves can be removed and their seat cleaned of carbon or debris. If the seal is worn, the spring is weak or the body is cracked, replace it. A new brass check valve is inexpensive, so replacement is usually the better choice.
Related problems that look similar
- A leaking unloader valve on the pressure switch can hiss like a bad check valve.
- Leaks at the drain valve, coupler or fittings drain the tank too; test with soapy water.
- A motor that won't start under load can also be a failing start capacitor or low voltage.
How to test an air compressor check valve
Check whether a compressor's check valve is leaking tank air back through the unloader.
You'll need: Safety glasses, Spray bottle of soapy water.
- Build pressure. Let the compressor run until it shuts off at cut-out pressure.
- Listen at the unloader. A short hiss from the pressure switch when the motor stops is normal. A hiss that continues for more than a few seconds suggests the check valve is leaking.
- Watch the tank gauge. With the outlet closed and no tools connected, note whether tank pressure keeps dropping over several minutes.
- Rule out other leaks. Spray soapy water on fittings, the drain valve and the coupler. Bubbles there mean a fitting leak, not necessarily a bad check valve.
- Depressurize before repair. Unplug the compressor and drain the tank completely before removing or replacing the check valve.
Frequently asked questions
Can a bad check valve stop a compressor from starting?
Yes. If the check valve leaks, the pump tries to start against tank pressure, so the motor hums, struggles or trips the breaker or thermal overload.
Is it safe to replace a check valve myself?
It's a common DIY repair if you unplug the compressor and drain the tank completely first. Never work on fittings while the tank holds pressure.
Where is the check valve on an air compressor?
Where the discharge tube from the pump enters the tank. The small tube from the pressure switch's unloader usually connects to it too.
Can a compressor work without a check valve?
No. Without it, tank air would flow back through the pump and the unloader, the tank wouldn't hold pressure and the motor would struggle to restart.