I just replaced the R/D to condenser hose on a coworker's W210 a couple weeks ago. Charge with dye first and get a blacklight going in pitch black - you will see exactly where it's leaking from. The hose is right in the line of fire of road debris, so it's easy to see why/how it fails. Aside from this hose, also check out the pressure relief in the R/D - refrigerant and PAG may spew from there if the aux fan(s) aren't turning (be sure to check fan operation).

Beyond that, the system is pretty stout and not leak prone. Of course the other obvious source of leaks is the condenser itself - I would check yours.

SHAMELESS PLUG: I happen to have a squeaky clean condenser for a 1997 E320 W210 that has straight fins with no bugs, no corrosion (TX car), has been flushed thoroughly in both directions, had vacuum pulled on it, and has been stored, boxed, and ready to ship for the last year. There haven't been any takers in the Parts forum, but I'm in no hurry. Let me know if you end up needing one.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280