'69 Mach 1 front discs still dragging/over heating after installing new flex lines.

 

Caliper pistons are sticking, but it's usually just one, not both at the same time...
The piston (pin) in the master cylinder could also not be allowing pressure release, fluid
to flow back into the reservoir from the front brake lines.

If they are floaters, the slide pins need replaced, but this again, is usually not both at
the same time.

I would guess it's the master cylinder at this point, for BOTH to be dragging. You
eliminated possible line collapse, so the master is left.

 

I had one of my calipers dragging and it was because I had installed it with a twist in the flex hose. Uncurled a simple twist and the problem went away.