If the tank check valve sticks shut or is heavily carboned, the pump is trying to push air into a dead end. Discharge line pressure climbs far above tank pressure, the intercooler or discharge line relief valve pops, the pump gets very hot, and the tank gauge barely moves.
1
Carbon buildup restricting flow through valve
highReciprocating
CheckInstall a test gauge in the discharge line ahead of the check valve (or use the unloader port). Compare to tank pressure while running: more than 10–15 psi difference means a restriction. Feel for an extremely hot discharge line and listen for the pump labouring.
FixRemove and clean or replace the check valve. Investigate why carbon formed: overheating pump, wrong oil (non-compressor oil or too light), overfilled crankcase, or worn rings pumping oil into the discharge.
ToolsTest gauge 0–300 psi · IR thermometer
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2
Valve installed backwards
highCheckLook for the flow arrow on the body. It must point toward the tank. Common after a DIY replacement.
FixReinstall in correct orientation.
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3
Ice in the check valve (cold climates)
medCheckUnit in an unheated shed below freezing, moisture-laden discharge. Valve body frosty, problem goes away after warming.
FixDrain tank daily, relocate or heat the compressor room, add a crankcase heater on outdoor units, and route the discharge line so it drains toward the tank.
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Safety
Lockout/tagout before opening enclosures. Verify zero energy. Bleed system pressure before opening any pneumatic connection.