I assume that you are getting a lot of metal particles on the face of the hall effect sensor. So, you need to investigate the source of the metal bits. Perhaps the brake rotors are rusted and shedding iron? Or, maybe you have pads that are very aggressive and are abetting this issue. I will also take a guess that you do a large percentage of driving in urban or stop and go traffic situations? If this is the case, you need a set of less aggressive brake pads. Cheers!