How is air getting into my lines?

I had a similar sitch with my 65 Dart. All the wheels bled fine except the left front. The brake line went from the "D" block down and around the frame member and up to the support clip; made a "U" bend down about an inch or so to the flex line connection. The flex line to the wheel cylinder had a slight uphill turn for it to flex. All these high points in the line to the wheel cylinder seemed to trap air I was constantly getting out, and then seemed to reappear. What I did was separate the flex at the hard line, bleed there and then re-connect. Then bleeding the flex line and wheel cylinder went much better. I used a hand operated pressure bleeder which made things easier than frustrating an assistant at the pedal; and I could pump to my heart's content. Do NOT trust anything out of the box for brakes. I've had wheel cylinders with metric fittings or bolts for domestic iron; I've found manufacturing debris in NEW parts, even ones that were made in the USA regardless of price or lifetime warranty. QA is just not dependable these days.