Brake trouble on an 81 KZ1000

I've been trying to chase down a problem and I'm out of ideas now.  Over the last couple years I've been having trouble with my rear brake, it feels like it's not releasing properly and seems to get worse as the pads wear down.  When I took apart the caliper there was a line of rust which I assumed was snagging the piston.  OK no problem, I grabbed another caliper off ebay this Spring and just replaced the whole unit.  Except now it's doing the same thing again only a couple hundred miles down the road, which makes me think I'm missing something.

I've also heard of something like this happening when there's contamination in the fluid line, but since I swapped the caliper the entire line was completely flushed with new fluid, and the old fluid didn't really look bad.  So it *should* be clean in there.

Any other suggestions to look for?  As it sits now, the brake is noticeably grabby even when cold (like it won't even roll back against the curb).  I ran to the store a few blocks away today and moving it around doesn't make it any better or worse.  The pedal moves freely and feels like it snaps back on its own without any trouble, and when I grab the pedal by hand it doesn't move up any further.  Since it's all hydraulic I don't think there were any adjustments to be made?  I'm not sure where to go from here.

Scale inside the old brake line will quickly contaminate any new fluid you flush though the system.  Also, you have to break down the master cylinder and clean it out, paying special attentio to the tiny 1mm fluid return hole because if it's plugged or restricted the brakes will stick ON.

Gah!  OK so then there's a chance the original unit may be suffering the same problem.  I'll have to dig up a diagram on where to find that return hole.

A question about scale getting into the fluid... what will this look like when I pump out the existing fluid?  What causes this to build up in a sealed line, and what can I do to clear it out?