1993-95 for most.
1994-1996 Depending on manufacturer.
There is no need to ever change the refrigerant which is R134 and not Freon R12.
They should never be mixed.
Initially used R12 . Almost impossible to get. Probably will have to change to R134
There is only one Freon, and that is R12. R12 Freon came installed in your 1991 Chevy. It may or may not have been changed over to R134 Refrigerant. If not change it over, as the cost of R12 is unbelievably high an is no longer in production.
94 is the year most vehicles were in the change-over stage from the factory from r-12 to r134, look for a yellow tag/stick under the hood as to which is in your system. It came from the factory with R12 Freon installed. Unless it has been converted to R134, then it still has R12. Due to the extremely high cost of R12, I suggest you convert it over to R134a.
Uses no Freon which is R12. Uses R134 refrigerant.
Freon is R12. Your 1997 Chevy uses R134 Refrigerant.
You do not want to buy Freon as it is very, very, expensive. Have the leak repaired, the system purged of all Freon R12 and the oil. Have it converted over to R134 refrigerant. You cannot just add R134 as the oil used in R12 is not compatible with R134. You will be money ahead by converting it.
Check the ports to add freon--the low side is a large hose --the high pressure is the small line--on the large line if the valve is the large style----the freon is R134--if the valve is the small style it is R12--no longer used --retro valves are available to convert R12 systems to R134 R12 fittings can't be connected to R134 gauges, and vice versa. If it's R12 you won't be able to connect the hose from the supply can to fill it, or do anything else with R134 equipment.
Only if you're trying to destroy your AC system. Never mix refrigerants.
it uses R12 but it should be converted to R134