To flush it, I filled the Harrison R4 that was removed from my 85 300D with 6 oz of mineral oil in addition to whatever PAG oil was in there, which wasn't much when I tried to pour it out, maybe a table spoon came out. Anyway with 6 oz of added oil, it did not hydrolock when I turned the compressor by hand. It just gurgled and spurted out the discharge line.
The other components have all been flushed and dried. I used a mix of carb cleaner and mineral spirit on the condenser and evaporator by cycling it continuously via a solenoid pump. The carb cleaner never touched any AC hoses since it contains some pretty strong solvents not resistant by the hoses, which I believe is Neoprene. To flush the hoses and degrease the condenser and evaporator, I cycled potasium hydroxide through it with the solenoid pump, followed by a few gallons of water, then blowing it out with compressed air, followed by boiling it out with a vacuum pump. We shall see how this works out. I'll have 2 more weeks for it to dry out in this hot weather (I have to order a TXV, dryer and PAO 68 oil) plus I will run the vac pump some more.
The solenoid pump made flushing neat with no mess no spills. This one is doing the evaporator by itself
https://www.youtube.com/watch?v=EcP8LJ90mAQ