I just bought some stainless lines for my 190d (they just came in today). I got mine from StopTech. I'm not sure if those will fit the 300, but I'd think if they have them for the 190 they'd have them for the 300. They can't be that much different.

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2010 Mercedes E350 4Matic - 77k miles
1986.5 Nissan D21 V6 4x4 - 276k miles
1987 Mercedes 190D 2.5 Turbo - 3??k miles
1983 Mercedes 300SD turbo parts car
2017 Honda CBR500R - 15k miles
2016 Suzuki GSX S1000 - 9k miles