Anyone know the size? Material? Can I find a similar part at home depot?

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1985 300d Turbo Diesel

I'm not sure about Home Depot but many parts chainstores carry them. Just ask for copper brake system washers or you will likely get a blank stare if you mention Mercedes.
IIRC, it should be an M8.

https://www.belmetric.com/banjo-fitting-gasket-c-11_937/bnjgsc8x12-banjo-fitting-gasket-p-5012.html

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Sam

84 300SD 350K+ miles ( Blue Belle )

thanks

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1985 300d Turbo Diesel