Sounds like a problem with the inlet valve. This valve is situated on the air inlet of the compressor. When it is closed, the compressor is running unloaded since no air can be sucked in. This is the mode for startup, stop en when maximum pressure is reached.
The inlet valve is opened and closed by control air, and this air is controlled by the central controller through a solenoid valve (the 'loading solenoid'). Please check all these parts.
Does the engine rev up when you press 'load'?
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