Wow, you did try everything... I'm assuming there's no leaking in the caliper nor the cylinder...
It is a fluctuating or solid-fixed disk? If it is the first case, maybe there's too much free play...
Are you using DOT 4 or higher brake fluid? Try to replace the fluid with a new one, from a brand-new, sealed container. These fluids are very sensitive to humidity (they absorbe the moisture from the air, and loose its characteristics).
I give up. This is too hard to find out... Sorry...