Emglo single Stage Wheeled Portable Electric Motor Air Compressor problem

My Emglo single Stage Wheeled Portable Electric Motor Air Compressor is giving me a problem. It runs and starts to build up pressure, then it seems to bog down and just stop compressing. The electric motor tries to keep compressing but I can't get much past 40 lbs. anymore. I replaced the pilot valve but that didn't help much. I had to adjust the pilot valve to the point that it disengages the compressor from the clutch on the motor.
Any ideas?
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I suspect that the one way check valve is not working. This check valve should keep the pressure off of the piston opening only to let air into the tank .

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Can you tell me where this valve is located?

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