Originally Posted by mr.smith.pdx

Instead of reaching back and opening the caliper while riding, grab the brake cable under the top tube, then pull the brake cable towards the back of the bike.

My rear Ultegra does the same thing. I have narrowed it down to where the ferrule on the brake cable meets the cable guide braze ons on the frame. The braze ons are too large for the ferrule, the ferrule can angle sideways and the cable get's pinched between the braze on and the opening to the ferrule. Too much friction for the return spring to overcome. I get slack in the cable under the top tube. THis helped me figure out where it was binding.

I discussed this with theh LBS and they said I could, with the ferrule in the braze on GENTLY squeeze it with pliers to pinch the braze on enough to hold the ferrule snugly.

I haven't done this yet because I don't want to crack the braze ons off the frame. I also may look for a larger diameter ferrule or one with a thicker wall.