have a 1996 buick park avenue. my question is.... do I have to evacuate the a/c system before I add new Freon??

In a word, YES! You do not, ever, open the system without evacuating it. Doing so can cause severe injury or death. There is bound to be some residual pressure in the system, and that will send what ever you open flying at high speed. When you recharge the system, you are supposed to change the dryer anyway, which means opening the system. Not changing the dryer compromised the performance of the system.

If your car A/C uses R12 Freon it's not available for the public to buy. And it is highly illegal to release it in to the atmosphere. So you have a few options, it may already use R134, it was prohibited in 1997 but many car manufacturers had already re engineered the A/C and you can just go ahead and recharge it from the low side port. If it has Freon in it, (Owners manual) it can be converted to R134 or R134a, if you are a DIY person great, buy the kit and re charge. If it has R12 it must be captured and disposed of properly...your car is right at the year it could have gone either way.

2 good answers- I would like to emphasize David's point- and I hope others read this- releasing Freon into the atmosphere is destroying the Earth's ozone layer- it is illegal, and immoral- professional repair shops spend thousands of dollars on the proper equipment to do this- so, yes, they have to recoup their expenses when a person needs to get their AC worked on- PLEASE- take the time to have a professional with the proper equipment do the work- if it is expensive, remember- you are supporting people who spend the money to buy the equipment to care for the planet- for your sake, my sake, our children's sake

The dryers are meant to be changed any time the system is in need of a recharge or any other time the system is opened. Simply put, opening the system without evacuating it is a good way to end up in the hospital or morgue. Yes, R12 is terrible for the environment. R134a is not that much better, after all. It too needs to be contained. However, environmental arguments seldom work. Personal safety stands a better chance of getting the message across. Unfortunately.

Michael, thank you for pointing out the damage done by all cooling agents. You are right that people just don't seem to care about the damage done to the earth. The point I wanted to make was that professional, RESPONSIBLE mechanics and shops now have equipment to capture these poisons. And IRRESPONSIBLE people still don't want to pony up the extra bucks, or take the time to stay cool the right way. They release poisons into the atmosphere. This is no different that pouring antifreeze or used motor oil down the storm drains in the street. WAKE UP people! YOU CAN'T RIDE FOR FREE

it really depends on what they mean by adding Freon. if it just needs a pound of Freon added, you can do that very easy. if they are changing from r12 then it needs to be done by a pro. there is NO danger to simply adding Freon to an ac system.