I had the exact same problem. 2011 HS33, standard magura clamps. I'd set it up perfect, tighten the clamps tight ( too tight as i stripped a thread today ) and by the end of a ride the pistons will have pushed right out and the lever would come into the bars. What I did was file the bottom of the clamps down so that they meet flat ( filed away the little bits on the end ) and now its perfect.