To be absolutely certain:
* Consult your Buick Century's repair manual: This is the most reliable source for the exact location and port identification.
* Use a refrigerant pressure gauge set: A properly calibrated gauge set will allow you to identify the low-pressure port by its lower pressure reading when the AC is running. Never attempt this without proper training and equipment. Improper handling of refrigerants can be dangerous.
Warning: Working on an automotive air conditioning system requires specialized knowledge, tools, and safety precautions. If you are not experienced with AC repair, it is strongly recommended that you take your vehicle to a qualified professional for diagnosis and repair. Improper handling of refrigerants can lead to injury or environmental damage.