A tank that empties while the compressor is off has a leak somewhere between the tank and the closed valves. It's almost never the pump.
1
Leaking fittings, drain valve or gauge
lowCheckPressurise, switch off, and soap-test every tank connection, drain, gauge and safety valve.
FixRe-seal or replace leaking parts.
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2
Check valve leaking back through unloader
medReciprocating
CheckHiss at the pressure switch unloader after shutdown that doesn't stop.
FixClean/replace check valve.
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3
Safety valve weeping
medCheckSoap at the safety valve outlet.
FixPull the ring briefly to reseat; replace if it continues. Never plug.
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4
Downstream leaks (if service valve left open)
lowCheckClose service valve overnight — if tank then holds, leaks are in the system.
FixLeak audit downstream.
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Safety
Lockout/tagout before opening enclosures. Verify zero energy. Bleed system pressure before opening any pneumatic connection.