On a recip/tank-mount unit, air should only bleed from the pressure-switch unloader for 1–3 seconds after the motor stops — just long enough to empty the discharge line. A continuous hiss that lasts until the tank drains, or the motor struggling to restart against pressure, means the tank check valve is letting stored air flow backward into the discharge line and out the unloader.
1
Carbon or debris holding the check valve disc/poppet open
medReciprocating
CheckRun the tank to cut-out and stop. Listen at the pressure-switch unloader: a 1–3 sec puff is normal; a steady hiss that does not stop is backflow. Confirm by watching the tank gauge — it will steadily fall while the hiss continues. Feel the discharge tube at the check valve: if it stays warm and pressurised long after shutdown, the valve is not seating.
FixBleed the tank to 0 psi and LOTO. Remove the unloader tube and discharge line, then unscrew the check valve (most unscrew with a wrench on the hex body — note flow arrow direction). Remove the cap/plug and clean the disc and seat with a non-residue solvent and a soft brush. Inspect the seat for pitting. If the disc is hardened, cracked, or the seat is cut, replace the whole valve — they're inexpensive. Reinstall with fresh thread sealant (not on the first two threads) and arrow pointing INTO the tank.
ToolsWrenches · Thread sealant · Solvent/brush
SafetyFully drain tank to 0 psi before loosening any fitting — a check valve that is leaking back can still hold residual pressure behind it.
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2
Worn or heat-hardened check valve seal (rubber/PTFE disc)
medReciprocating
CheckRemove the valve and inspect the disc. Rubber discs exposed to high discharge temperature go glassy, cracked or flattened and can no longer seal. Hold the valve to your mouth (clean!) and try to blow backward through the tank side — you should not be able to pass air.
FixReplace the check valve. On units that run hot (high duty cycle, poor ventilation), upgrade to an all-metal or high-temp PTFE seat valve and lengthen the discharge tube to drop air temperature before it reaches the valve.
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3
Weak or broken return spring
lowCheckWith the valve removed, push the poppet off the seat — it should snap back firmly. If it's loose, slow to return, or the spring is broken, the valve won't close fast enough to stop backflow.
FixReplace the valve (spring kits rarely save money vs a new valve).
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4
Unloader tube connected to wrong port or unloader valve stuck open
lowReciprocating
CheckConfirm the small unloader tube runs from the check valve's side port (the discharge-line side, upstream of the disc) to the pressure-switch unloader. If teed downstream of the check valve it will drain the tank. Also check the unloader valve itself: if it hisses while the motor is running, its pin is stuck or the O-ring is bad (see the pressure-switch unloader flow).
FixRe-pipe the tube to the correct port; replace unloader valve or pressure switch if it doesn't close.
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Safety
Lockout/tagout before opening enclosures. Verify zero energy. Bleed system pressure before opening any pneumatic connection.