The problem with mercedes evaporators are they are made of copper and alluminum which is not a proper seal. You cant seal alluminum to copper properly. Now late model evaporators like on the 202 chasis cars are made from just alluminum I have seen these leak but not as much as earlier types. You can go with all copper , on early cars most are still r-12 so alluminum does just fine . On a late model which is 134 you are correct I would go with copper. It takes air in the system for corrosion to occur which you should not have any in an air conditioning system to begin with.

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euro 287
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck