There are 2 Schraeder valves on the R12 system (we're talking 123 chassis here). One is within a few inches of the thermostat on top of the a/c line that "returns" from the vehicle cabin. This is the "lo-side" valve where one of the guages is attached during diagnostics.

The "hi-side" fill valve is behind the alternator, its hard to see if you don't know where to look.

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)