shertex,

Since the AC system is under positive pressure, no contaminents will get in as long as it is not allowed to go completely empty. There is a pressure switch that will not allow the compressor clutch to turn the compressor if the pressure is to low.

So as long as you keep enough refrigerant in the system so that it functions, you should be OK adding refergerant as needed.

If you replaced the evaporator yourself, it should only cost the price of the evaporator and new refrigerant.

P E H