I second Larry's post. I will tell you my experience. I didn't pull the expansion valve last summer after a compressor failed from a 134a retrofit. My "new" compressor off my wrecked car only lasted 1000 miles.
This time I pulled the expansion valve so I could flush the evaporator and all the hoses. I used mineral spirits first since it is cheap. After thinking all the black oil was out of the condensor and evaporator I used some 141 flush. A great deal more "black oil' came out of what I thought was a flushed system.
You can not flush enough. I knew better last time, but took a short cut by not pulling the expansion valve and I had to pay for it.
The Duraflush or 141 flush is $$$$$ which is why I used the mineral spirits first. If you are going to do it, I recommend doing it right once.
New R4 new dryer and 2.2 oz (W116) of R12. It is now working great!
Good luck and be cool
