You might want to consider replacing the compressor, expansion valve and receiver/dryer, in the grand scheme of things they are not that expensive. I think the compressor on that car is a Delco A6, not sure. Then flush out the hoses, evaporator, and condensor as best you can with those items installed on the car.

For real good advice by A/C professionals go to:

http://www.autoacforum.com/categories.cfm?catid=2