Auto Air Conditioning & Heater Repair: AC Recharge!!!, vacuum pump, atmospheric air

Question
Hi, I have a 99 Expy EB 2wd.  I want to recharge my AC.  When I check the Low presure port with a gauge, do I have to have my car running with AC on?  And when I refill do I have to have my car running with AC on to?  Thanks for your help.  It is greatly appreciated....If you know what are the specs on refilling with 134a on my car and the presure.  

Answer
If you have to recharge the entire ac system you MUST evacuate the atmospheric air from the system BEFORE you recharge it using a vacuum pump or you won't get hardly any freon in it and it will NOT provide cold air.

The pressures vary dependent upon the atmospheric temperature and humidity of the day.