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Proper way to "pull" a vacuum from a discharged auto AC system. This applies to the newer R134a refrigerant systems as well as the older R12 systems.

It is not only necessary, but critical that you evacuate your AC system IF it has been opened to the atmoshere. However, if you are just adding additional R134A Freon to your system, evacuation is not required.

Automotive A/C Vacuum Pumps are used to remove air and moisture from an A/C system that has been worked on. If, for example, the A/C system has been repaired(compressor, condensor, evaporator, accummulator,or whatever), this is when evacuation is required. During evacuation, the air is removed from the a/c system and at 28" pounds of vacuum, the moisture begins to boil. The moisture now becomes vapor and is drawn out of the system by the vacuum pump.

At the end of the evacuation, you have a system that is free of any air or moisture. If you do not evacuate your system prior to charge, it may work for awhile, but the moisture will eventually cause corrosion and other unwanted chemical reactions in your A/C system. This will shorten the operating life of your A/C system.