How to Replace a Check Valve

A worn or stuck check valve often shows up as constant bleeding at the unloader and frequent cycling. Replacing it is a straightforward fix: shut power off, drain pressure, remove the old valve, install the new one with the correct orientation, then test for leaks and proper restart behavior. 

Why this matters 

A check valve keeps compressed air from flowing backward from the tank into the pump. When it can’t seal, air leaks back toward the pump and out the unloader circuit. That can sound like hissing at the unloader and can trigger nonstop cycling.  [555 | PDF] 

Common signs you’re dealing with the check valve: 

  • Hissing at the unloader after shutdown, that doesn’t stop after a short bleed-down.
  • Hard restarts (motor struggles because head pressure never fully unloads).
  • Short cycling as pressure bleeds back and the pressure switch keeps calling for air.

Safety first:

You’ll be working around stored energy (compressed air) and threaded fittings under stress. Do these every time: 

  1. Power OFF the unit at the switch and, if available, disconnect from power.
  2. Drain the tank fully until the gauge reads zero and no air comes out.
  3. Wear safety glasses and cut-resistant gloves.

If your compressor is hardwired or you’re not authorized to isolate power in your facility, route the job through a licensed electrician or your maintenance process. 

 

What you’ll need 

  • Adjustable wrench or combination wrenches
  • PTFE tape or thread sealant
  • Safety glasses and cut-resistant gloves 

Nice to have: small wire brush, shop rag, and a marker or phone photo to capture line routing before disassembly. 

Step-by-step: Replace the check valve 

  1. De-energize and depressurize
  • Turn Power OFF.
  • Drain the tank fully. Confirm the tank pressure gauge is at 0 psi and no air releases when you crack a fitting.  

Pro tip: Leave the tank drain open while you work. It prevents surprise pressure buildup.

   2. Disconnect lines from the check valve

At the check valve, disconnect:

  • Discharge tube (from pump head to tank)
  • Unloader tube
  • Pressure Relief Valve (PRV) if it’s mounted to the check valve [555 | PDF]

Avoid rework: Snap a quick photo before removing anything so you can restore the original routing.

  3. Remove the check valve and inspect it

  • Unscrew the check valve from the tank port.
  • Inspect for: 
  • A gummed poppet (sticky seal surface that won’t seat)
  • A weak spring or damaged sealing face that prevents closing 

If you see debris or sludge, wipe the tank port threads and clean the area so it doesn’t contaminate the new valve on install.

  4. Install the new check valve (orientation matters)

  • Apply PTFE tape or thread sealant to the male threads that go into the tank port.  
  • Thread the new valve into the tank by hand first to avoid cross-threading.
  • Tighten snugly with a wrench.

Key detail: Align the unloader port correctly so the unloader tube reaches without kinking or strain.  

If your PRV mounts on the check valve, reinstall it now.

Threading tip: PTFE tape works best when wrapped in the direction of tightening (so it doesn’t unravel as you torque the fitting).

  5. Reconnect lines and test

  • Reattach the discharge tube and unloader tube.
  • Restore power and run the unit.
  • Confirm the prior leak is gone, especially at shutdown. [555 | PDF]

What “good” looks like:

  • At shutdown, you may hear a brief unloader bleed, then silence.
  • No ongoing hiss from the unloader line.
  • The unit restarts without struggling.

Quick troubleshooting 

If you still hear constant hissing at the unloader 

  • Recheck that the new check valve is seated and sealed (thread sealant/tape, proper tightness). 
  • Inspect the unloader line for cracks or a loose fitting.
  • If the hissing is coming from a different location, the problem may be the pressure switch unloader rather than the check valve.

If you have leaks at the tank port threads 

  • Remove the valve, clean threads, reapply PTFE tape/sealant, reinstall. 

If the unit struggles to restart 

  • Confirm the unloader line is connected to the correct port and not blocked.
  • Verify the check valve unloader port is oriented correctly and the tube isn’t kinked.

FAQS

If air continuously bleeds from the unloader after shutdown, a leaking check valve is a common cause because tank air is pushing back through the valve into the unloader circuit.  
If the air bleed is only momentary at shutdown, that’s normal unloader operation.

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