dclinard,
If you are new to 850, they were/are famous for evaporator failures. The system back then used the new R134a refrigerent not R12. Your choice is to have the expensive evaporator replacement or re-charge only. Re-charging only works providing you have a very slow leak. I have re-charged my 850 every early summer for the last three years. I have a very slow leak that holds a charge for approx 6 months. I'm too cheap to spend the big bucks on an evaporator replacement. BTW, some jurisdictions do not allow re-charging by law. For example here in Ontario, it is illegal to re-charge a system (auto,home,refrigerator) with a known leak. Yes, I break the law every time I re-charge.