If the leak is right by the attachment nut, that is usually caused by overtightening of the nut. That causes the ferrule to overstress the hardline and vibration cracks often result. However the hard lines can leak right in the middle, as I had one on the 220D develop a very small pin-hole which saturated the engine compartment. We were on a long trip and had to have it brazed in a small farm town in southern Wyoming.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150