I was told to always flush a system that has had a compressor failure or when retrofitting to a different Freon. You can buy a can of flushing solvent with the hose attached in an aerosol type can at parts stores. Autozone and O'Rileys have it for sure. Not much flush needed to do hoses and parts. Just flush through so if comes out the other side and stop and let it evaporate. It works fast. Kind of like brake cleaner that evaporates.

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red