Air compressor check valve , unloaded questions

[Bonestock3ott]

I recently picked up an old air compressor off of craigs list and when using it for about 30 minutes to air up the tires of my 7 vehicles it started tripping the breaker. 20 Amp outlet, nothing else on that circuit. My question is the compressor has a pressure switch without a unloader line and only one line off the cylinder head to the check valve. Does anyone know if older compressors utilized a different method of relieving head pressure to aid motor restarting? When the compressor is running it leaks at the check valve but does not leak when motor is off is that proper?

[Bonestock3ott]

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