I got a single sided 6 piston on my 13" rotor and the other day I felt the bike slow down really fast. I grabbed a fist full of front brake and let it go really fast and it seemed like it went away. I got 700 miles to do over the course of the weekend so I decided just to check it. The rotor has small grooves from the caliper dragging. I got a 13 ultra and I'm unsure what to try first. I bled the brakes once and next up is shims but I'm unsure if the caliper is actually returning to its original state after I grab the front brake.

 

I had 6-pot calipers on my Glide, with 13" rotors, until recently. Mine were custom ones and often troublesome, especially with sticking pistons. I suggest you dismount yours and give it a thorough clean, making sure all the pistons move freely. It only requires one of them to rebel and cause the problem you have.