I am working on a 52 Chevy Styleline, at least untill my knee replacement, and my brake lines were like yours, a combination of 3/16 and 1/4 lines. I have no idea why!!!! I used only 3/16 to plumb it now. Haven't done any bleeding on it yet though, too much else to do first. My 53 ad pickup had all 3/16 lines, but it was not on an original frame when I bought it. Hope that helps!
Coach Frank

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72 Nova SS sold
75 Corvette
52 Chevy Styleline Coupe