what manufactors freeon in this vehicle
Check the sticker on the inside of the engine compartment. It should list it there. Since your truck is a '91, I suspect you have R12 which is not used anymore. The port will also be a screw type schrader valve. If you need some AC work, a proper shop can evacuate the system and add add/convert to the R134a, which is the current standard of refrigerant.
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