I am glag you got rid of the codes, makes things easier to diagnose.

When testing the r134a pressure, the engine and compressor must be running. Do not turn the can up side down! The can needs to be warm, just shake it before starting. If the oil doesn't slap around in the can then the can is empty. If the can is cold, it won't work.

Turn the temperature to coldest setting on the dash. Add r134a with the compressor running. At the firewall, when both AC lines get cold, then you have enough fluid. If you have too much pressure, the system has a pressure relief valve which goes off with a bang! No harm done, but it sure is nerve wracking.

The compressor should turn on as soon as you hook up a bottle of r134a. If not, then unplug the pressostat (black round thing on the AC piping close to the firewall) and short the plug with a paperclip. Shorting the switch should immediately start the compressor. Don't do this for more than a few minutes.

The S belt should last as long as the timing belt (70K). Check it for cracking. If it were slipping around the compressor, you would hear the terrible noise.

Also try using a hose and spray nozzle to clean out the fins on the radiator and condenser, especially near the bottom. Over the years a lot of grime collects and restricts air movement.

Klaus
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Why are Volvos so endearing? Its just a car.