Rear brake line questions/first post!

[65'RustBucket]

Hello, I just recently acquired a 1965 c10 longbed and have been running into some issues with fitment for my steel brake lines in the rear. All the prefab ones that I have tried seem to not have a "flat" enough double flange which, when fluid is pumped through, does not create a proper seal to both the wheel cylinder and T joint and I also suspect the culprit behind the reason it was so difficult to get the POS to catch threads. To clarify, it is 3/16ths in the rear? And has anyone else run into this issue with the prefab lines? Is there a special connector/type of flange for this truck? Am I doing something wrong? Haha it seems like a pretty straightforward task... And final question, should I just make my own lines instead? I see that they have kits at NAPA, has anyone used this hardware with luck? Thanks again!
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