[RichardJ]

>>Blow all the lines out good and whatever you do pull a good vacuum <<

It has been mentioned a couple of times about blowing the lines out.
You really should use a AC Flush Solvent. Pour it through in both directions and let it soak for awhile. The lines need to be flushed as well.

You don't pour it through the compressor or the Accumulator or Dryer depending on the system.

If changing from R12 with mineral oil to 134, I pour ester oil into the compressor (off the engine) and turn the compressor by hand until all the old mineral oil and the Ester oil has been flushed and drained out.
Then add the correct amount of new ester oil to the system.

You should be careful blowing out the hoses, evap and condenser if you have an old air compressor the uses a lot of oil. My air compressor is in good shape, but I make sure the compressor hasn't run in several hours before I blow the excess Flush out.