Depending on the system design and BTU capacity, retrofitting to R134a from R12 will be anywhere from 70-95% of the charge weight of R12. The vast majority fall in the 80-90% range.

You can flush the system if you want. While you have it apart, replace every O-ring in the system and use Nylog (blue bottle) smeared on the O-rings. Remove the expansion valve while you have it apart, it needs to come out to flush the system anyway. After you flush, blow everything out with compressed air until nothing else comes out.

When you put the system back together, pump it down on the vacuum pump for a minimum of 1 hour to make sure you have a nice dry system. Keep in mind that the dye will stain the oil residues in the compressor, so when you recharge, you'll still have some yellow dye in there.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]