It is about the unloader valve of mobile air compressor.

The air compressor is not loading above 4 bar.

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Bill says...

G'day.

You say... "It is about the unloader valve of mobile air compressor" and then "...The air compressor is not loading above 4 bar."

A bit confusing.

I take it to mean that your compressor runs, and that the pressure in the tank won't exceed 4 bar or about 60 PSI.

That being the case, please have a look at this page about why compressors won't build pressure.

Cheers,

Bill