I took a "toothbrush type" wire brush and knocked loose any external rust so that the penetrant could soak in. I then used a mix of 1/2 Acetone and 1/2 trans fluid and let it soak. I was able to break loose 13 45 year old brake lines and reuse the original lines.

Let it sit a day or two and keep squirting it in.

1972 Cuda 340 4 speed, 2001 Ram CTD 4x4 6 speed, 1970 Duster 408 4 speed, 1996 Ram 5.9 2x4 auto, 1965 Coronet 500