I have 4 wheel anti-lock brakes on my 1998 ford expy that keep coming on even if it is not slippery. I just touch the brakes a little and the anti-lock kick in (the brake begins to vibrate and I hear a sound). I live in a very snowy and icey place and they are used very often (almost every time I stop), but now they don't want to stop. Does anyone know whats going on?