That's an old tank! They tend to lose their vacuum over time and start to fail.
From our website
http://sementank.com/
Signs of a failing tank--
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The time between fills is becoming shorter.
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You notice an increase in liquid nitrogen usage.
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You notice frost forming around the neck.
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You notice condensation forming on the outside of the tank.
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The vacuum plug is missing or is loose in the vacuum port.
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The tank has a sharp ping or a thud to it when tapped.