Originally Posted by H96669
Ah....maybe just the pressure relief hole inside the reservoir is plugged.That hole sure is tiny on some systems. You may be able to see it when you remove the cover. Then carefully pull/release the brake, you should see pressure coming out of the hole as you release the lever. Watch out, may splash up and shoot brake fluid where you don't want it, like your eyes or painted surfaces.
If you open the bleeder screw and some fluid squirts out and/or the pads are no longer dragging, the the relief hole is the problem.