Lack of nitrogen purge
Brazing requires use of extreme heat to melt joining material. The heat and burning acetylene reacts with the oxygen in the air to leave deposits of black "soot" on both the inside and outside of the piping. Best practices requires the use of a nitrogen purge to bleed nitrogen gas into the piping system to push the oxygen from the piping to prevent oxidation on the inside of the pipe.
Since contamination of the oil and refrigerant is the most common cause of compressor failure, the nitrogen purge is likely one of the most important steps in the clean installation of a refrigerant pipe system.
Unfortunately, skipping the nitrogen purge is the most common short cut found in most installatios.
Brazing with and without nitrogen purge.
If your installer skips the nitrogen purge, the inside of the piping is coated in black soot as you can see below.
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