‘Purge the system’ means flushing each component; condenser, evaporator core, individual hoses and the compressor of the old oil and residue prior to refilling with the new oil and 134. The compressor cannot be flushed without removing it and either pulling off the back plate and cleaning out the old oil or possibly flushing through both fill port bolts, top and bottom. When I did my flush, I found considerable oil in the evaporator core. Apparently, if you have a slow Freon leak over time, the oil will drop out inside the evap core and remain stagnant. The TXV ( thermal expansion valve) located at the firewall where the evap core lines exit cannot be flushed but must be removed to access the evap core lines for flushing. The filter dryer next to the condenser must also not be flushed, but replaced. The desiccant inside is apparently degraded by the flushing solvent. Flushing is made easier by using O’Reilly’s tool loaner program and ‘borrowing’ their flush tool. A/C Flushing solvent is not cheap, but is safer than alternatives like lacquer thinner due to the highly flammable nature of that stuff. That said, I used lacquer thinner followed by a ‘rinse’ with the expensive stuff. If you do the flushing outside the flammability issue is lessened. I attached a 5’ section of clear vinyl tubing to the ‘exhaust’ end to keep the lacquer thinner away from the under hood paint. I purged with LOTS of shop air to thoroughly remove all traces of solvent, as it will degrade the new oil. The old R12 system uses mineral oil and the 134 system uses a PAG oil. I took about 4 hours to flush the lines and components on my system, the TXV removal is a PITA…
