What type of refrigerant goes into a 1991 Chevy K1500?

A 1991 Chevy K1500 likely used R12 (Freon-12) refrigerant in its air conditioning system. However, R12 is no longer produced due to its ozone-depleting properties. Any repairs or recharge would necessitate using an approved R134a retrofit kit and refrigerant. You should never attempt to add R12 to a vehicle's AC system.