Service technicians need to completely evacuate a refrigeration system any time they open the system to the atmosphere. Vacuum pumps and micron gauges are the tools you need to use for this. Are yours in good working order?
System evacuation is just good service common sense. You need to remove any water vapor, nitrogen, and oxygen contained in the air before you can add refrigerant. If you leave air in the system, it will collect in the condenser, taking up space and causing the high-side pressure to be higher than normal.