Yes, you need to pull a vacuum on the system once you connect it back up. It will remove the air will also help you check for leaks. This will prevent contamination of the system. IF you get an air pocket in the system you will see an increase in pressure while the system runs. It will rise up and fall as the air passes through. You can vacuum the system with a machine or gauge set with a vacuum pump. The best way is with an R134a A/C machine. You can then put the correct weight of refrigerant in the system and oil, if needed. If you have a good amount of miles on the car, It is a good idea to replace the receiver dryer. This is the filter for the system. It filters out all of the moisture and trash.
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