I suppose that I overlooked the filter/drier. To me it goes without saying that you replace it whenever doing massive open surgery such as completely opening the system, flushing and replacing the system.
Additionally, you need to evacuate the system thoroughly. If you leave any traces of moisture, it combines with the refrigerant to create an acid which will eat through evaporartors and such. This is much worse in an R134 system. I have a very good vacuum pump now, and on an R134 system I prefer to pump down for 24 hours before charging the system.
Best of luck,