Most new backdrafts are built with certain used parts with unknown time and mileage that are just cleaned up and sometimes painted and then installed. I had a brake problem at very low miles from this unfortunate fact...an internal caliper o-ring and a brake failure on the right front of my car. When I took it off and repaired it it was obviously very "used". This being said, there could be a build up of decomposing rubber and gunk that will keep the piston from fully returning. If you have also rubber lines, sometimes they will decompose internally, allowing fluid to move one way and putting a resistance on the return flow and hanging the brakes a bit. Those are the only two things that I have seen hang a brake pedal.
Other than that, spray some wd 40 on all of your pedal joints!
