It's definitely the cylinder. There's washers on both sides and I even lapped them. I took it apart again last night and this time I checked the clutch pushrod...wasn't perfectly straight. I straightened it (probably won't work, but sometimes I get lucky with this kind of stuff) and reinstalled it, so as soon as I get a chance to ride I guess I'll find out.

You're right, thought, the slave and piston are pretty simple parts. I think what was happening was the out of round pushrod was making the piston wobble inside the cylinder, perhaps "pumping" fluid past the seals. We'll see...