Interesting drum brake problem

Ditto again on the old brakes lines; this will cause the brake dragging every time. The inside liner breaks down, and a chunk pushes into the passage and blocks the return flow of the fluid; it is amazing how much pressure this can hold on the cylinder or caliper.

In reading of your brake symptoms and the occasional brake light, I am highly suspicious that the former owner did not properly bench bleed the new master cylinder before installing. I would be taking it off, bench bleeding it throughly, and starting all over in a complete bleed of the other lines.

BTW, you ought to be putting a bit of lube on the backplates where the edges of the brake pads rub.

Have you put on any new spring kits, especially on the back where the one brake has been seizing up?