It's not just air which you want to get out of there. Probably more important is MOISTURE, because the moisture can and will freeze as it goes through the orifice / TXV and PLUGS the thing as it freezes. This causes interesting, aggravating intermittent problems.
Look at a chart of the boiling point of water. It's NOT 212F, you know, if it's NOT at "standard" atmospheric pressure
http://www.jbind.com/pdf/cross-reference-of-boiling-temps.pdf
At 20" Hg, the boiling point has dropped to about 160F.
But if you can get the vacuum down to 28---barely 29" Hg, the boiling point drops clear down to 75-80F
And if you can get it down to 29.8---on to 29.9" Hg, NOW the boiling point is down into the low 20F range and INTO BELOW ZERO!!!
What does this mean?? Simple. It means moisture in the system is going to boil off more easily and be carried off by the evacuation. So you can HELP this evacuation by doing these things........
1....Evacuate the system, if possible, on a warm day
2....If the car can be run, do so and get the engine up to temperature, and run the heater as well. This will get heat into the AC system, helping to boil off moisture.
Also, "in the day" before refrigerant recovery was "the law" it used to be common to run a "sweep charge" through the system, sometimes, more than once. How do you do this?
Think before you do something. Plan it.
Arrange to feed refrigerant INTO the high side fitting. This will "sweep" through the system just as if the compressor was driving it, fed by the pressure in the refrigerant container. This will push moisture and air out of the system.
To do this you need a way to VENT this. So you use a valve or second AC manifold so that as the low side comes out of a vacuum, and comes up "positive" in pressure, you OPEN the low side compressor fitting and allow the sweep charge to vent.
Of course "all of this is in the past," right? RIGHT!!!???