It looked like I was about to finally end the most epic simple brake job of all-time, and sure enough, one of the brake lines snapped in the rear when I went to replace the hose. The steel lines look plain wretched back there and need to be replaced; the front ones look fantastic. I do have some rear brake lines from a parts car that look quite good, that have been sitting in my shed. Any reason not to trust used hard brake lines that look perfect, assuming they are thoroughly flushed?

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)