If you're filling from a DIY kit (in other words a little can), then you want to add refrigerant on the low pressure side of the system. Trace the lines. The high pressure side runs from the compressor to the condensor (radiator-like thing in front of the radiator, probably through a reciever/dryer). The low pressure side starts at the metering valve (or orifice tube, doubtful on this vintage), through the evaporator (another radiator-like thing, but typically buried in the dash and in line to cool incoming air for the cab), and back out to the compressor.

It sounds like you've located a pressure port for the high pressure side. That's not the one you want.

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83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
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