Various things could cause a coolant leak: cracked upper radiator connection, especially if it is the original radiator; leaking water pump which needs to be replaced; leaking hose connections, etc. If it passes your cursory inspection and you are still interested in buying this 500SL, recommend having a MB-trained tech who is familiar with this vintage MB vehicle perform a PPI (pre-purchase inspection). That way you will know what major repairs are pending.

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Fred Hoelzle