I have been fighting this problem for a while now, as well. I clean the pistons as described earlier and it does work...For a while. I have also spoken to a few dealers about this and they almost always look at you like you're an idiot. I always ask them why HD switched to Brembos then? To save money? Yeah right. I do find it interesting that there doesn't seem to be the same problem with single disc systems. I had a Softail and have many friends with Softails and Dynas with the same caliper and they don't have the same problems. It does seem like the only permanent fix is to replace the system with the new Brembos, PMs or other aftermarket. But, which one?

 

Russell brake lines - eliminated any brake fade.
Floating rotors - eliminated uneven pad wear
I suspect HD switched to Brembo brakes for the same reason they make changes every year - somtimes for marketing purposes, sometimes to obey federal mandates, sometimes to save money, sometimes to improve product performance.

 

I am not familiar with the latest calipers (although I fitted one to the rear of my Glide recently it works OK at present), but over the years I have found on some bikes that the pads themselves can also hang up, due to build up of dust inside the caliper. So as well as checking movement of all the pistons, make certain the pads can move freely as well.