Im not sure what the resistance values should be for the abs sensor on the hub assembly but if you unplug each one, one at a time and measure the values I'm positive that 1 will be considerably different than the other 3. Just a thought for quick trouble shooting!
Back to the filter change or flush, if the filters doing what it's supposed to there should be no reason for there to be anything in the lines or the cooler and anything metallic will get picked up by the magnet in the pan.