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The bypass is to allow the compressor to start with low oil pressure (you may know that already). So you could have a faulty pressure sending unit or worst case low oil pressure.

 

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The low oil pressure cut-off switch should be for the engine. If the engine has good oil pressure, then you may want to look at the cut-off switch.

On my compressors there is a pressure gauge on top of the hydraulic reservoir/pressure tank. It's the large round cylinder near the air compressor housing.