Where is the air conditioning coolant refill port located on a 1998 Chevy Lumina?

I see two low pressure ports and one high pressure port on my 1998 Chevy Lumina sedan. The quick connect high and low pressure ports are both on the driver's side of the engine UNDER the air cleaner. You may have to remove the air filter and move the box around to access them. There is also a low pressure service port on a small diameter tube on the drivers side up high and against the firewall. It looks like a conventional screw-on type connector similar to a tire valve.