When doing AC charging the system must be evacuated of all air. If any
air is in the system you will be screwing up your compressor. The system
will also not be running at optimal cooling temperature. It does not
matter if you only opened the lines for 20 minutes or 1 second. If air
bleeds in moisture and air will contaminate your refrigerant. The
compressor will not last long either. There are also things you have to
watch out for when you charge like not filling liquid in to the system
with the compressor running. This is a common mistake for people who like
to rush charging the system. Hope this saves some of your ACs. Aloha, Ed