A 1991 Chevy pickup truck's air conditioning system most likely used
R-12 (Freon-12). However, it's crucial to
check the sticker on the underside of the hood or in the glove compartment. This sticker will definitively state the refrigerant used in that specific vehicle.
R-12 is no longer manufactured due to its ozone-depleting properties. If it needs recharging, it's illegal to use R-12 and it's likely the system will need to be converted to use a different refrigerant, such as R-134a, which requires some system modifications.
