I had an attack rear set do a similar thing. It feels like the rear brake isn't fully disengaged. It isn't an issue really, but there might be some ways to work around it. I start by keeping the pivot areas of all levers that work off the rear sets oiled with a light weight oil or silicon spray. Less friction at those pivot points helps a little. You might be able to tap a small hole into the lever and another spot and rig a small spring in there to act as a return spring.
03 RC51 for street duty/ 00 RC51 for track duty