Once upon a time I flushed an AC system. The solvent that was used is extremely nasty stuff. You don't want to inhale much of it, and safe disposal may not be easy. The flushing was straightforward. I disconnected the system in a few places and flushed part of it at a time to minimize pooling in low points.
I don't recall enough to steer you accurately, other than to say "Google it". There were a few online tutorials and FAQs that helped with the oil question.
Out of curiosity, are you going with R-12 or converting to R-134a when you have it recharged? Do you have your own vacuum pump and manifold setup for refilling it?
Procedure Arizona Mobile Air Inc. follows when flushing a system to remove debris and oil from an automotive air-conditioning system.
1. Each individual component needs to be flushed separately. Removal of hoses or any other devices may be needed for proper flushing.
2. Flush system with either an A/C flushing agent or products such as Mineral Sprits, Hecat Safe Flush and Paint Thinner. Careful with Paint Thinner for obvious reasons. Helping to get the flushing agaents through the system componets are the Mastercool Flush Gun Kit or for the professional the Hecat Professional Flush Equipment. We do not recommend aerosol flushes as they are not cost effective in our opinion. After using flushing agent use compressed air to blow any residue left behind from the flushing process. It also helps to place you finger or a shop rag on the opposite end to create some back pressure to dislodge any debris. It's best to perform this procedure many times in both directions to achieve the cleanest system possible. After you think its clean do it again you can never have a clean enough system. Some condensers are very difficult to flush and may need to be replaced. Cheaper replacing them then to have debris and burnt oil left in the system and damage new components.
3. Do not flush any of these components accumulator/drier, compressors, expansion valves/orifice tubes and any hose assembles the have either a muffler or filter attached to them. These items need to be replaced.
4. Our company will use Nitrogen after the flushing stage as a finial flush to remove any moisture that maybe left behind by the shop air.
5. At this point you are ready to replace the damage components along with correct oil type and amount. System should be evacuated and recharged per OEM guidelines. Evacuation should be done for a minimum of one hour. Charging capacities very from vehicle to vehicle. So it is best charge to OEM listed capacity when available! Conversions can be changed by pressures trying to achieve as close to the OEM R12 capacity without creating a high pressure issue. Complete set of refrigerant gauges are needed for this as you will need both low and high side pressures to perform these tasks.
