It only needs to be evacuated, if it has been open to the atmosphere such as changing out a part, or if the leak was severe enough that it not only leaked out the freon but with pressure changes sucked air in. Generally with a leak (unless it is severe) some pressure remains in the system and air can't get in, in this case it only needs the freon, leaking systems also lose refrigerate oil which must be replaced, otherwise it is like running your car low on oil, eventually it will end in failure. If you are only adding 1 maybe 2 cns or lbs of freon a year, thats not too severe for such an old car, it may need to have the o-rings replaced unless someone did them when they converted to 134, they should have also added the oil compatible with 134 as that time. You can put in your own 134, you can buy it any auto parts store, even some W-marts carry it, you can also buy the adapter hose to add it, I would get at the auto parts store, they should have one with a metal end, the cheap ones have a plastic end and won't last. Freon needs to be added on the low side, this is the fitting on the compressor that leads from the firewall. If you are going to add the freon your self, let us know. We or I will provide additional instructions. It is not difficult. Post your results. Good Luck.