In earlier years, these systems were flushed with the popular CFC-based solvent R-11. Non-flammable, plastic-safe, and with a very low boiling point, this material was favored because it was safe to use, cleaned efficiently, and was easily removed from the system.
Unfortunately, R-11 was a major ozone depleter and was gradually phased out of production. Since that time, many companies have attempted to market alternative flushing agents based on terpene, hydrocarbon cleaners, acid flushes, and oil additives.