Builds pressure to 170 (should be 175) but after a couple hours with line valve closed its down to 155 . Service tech can’t find anything wrong. Its brand new .
What do you mean by ‘after a couple hours with line valve closed its down to 155’ ? Is it slowly leaking air?
Did you try to adjust the pressure switch? The compressor should stop at the set pressure, but in reality it can go a bit higher.
If the pressure drops slowly, you have a leak somewhere. There’s a check valve between the compressor and the tank, it could be leaking, but it’s not a very big of a problem.
Hope this helps,