Message from the peanut gallery.
I can't speak to the Amazon disc brakes specifically, but based on my experience with an MGA, my guess is that your piston seals are old and hard and need replacement. So, a sticking caliper issue. On my MGA brakes, the piston seal and dust seal flex when the pistons move forward to brake, and when pressure is off the brake pedal, the seals flex backwards and pull the piston back. The brake pad motion is quite small.
If your problem is deteriorating brake lines, you can diagnose that by cracking the bleeder and letting a little fluid out. If that is sufficient to relieve the pressure so the pads retract, then the problem is probably in the lines - or at least upstream.
A third option might be that a relief "bypass" in the master cylinder is blocked.
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