Originally Posted by Mike

My theory is that when the brakes get hot, the caliper expands slightly and allows just a trace of fluid to leak out around the threads. On the track, I got the stock calipers hot enough to melt the powdercoat on my wheels and they can even get fairly hot with aggressive street or mountain driving.

I get this even on my stoptechs, but they are anodized, so it doesn't stain them.