Have you checked that the pistons and sliders are freely moving in the front calipers? A problem there could be causing a brake imbalance. Also are all discs and pads of matching condition, are they matching pairs? Either new or in good condition if secondhand. I'm not trying to insult you with this question, but I actually met a guy a while back who told me that his car was behaving badly after changing his front passengers disc. I didnt even bother to try and explain. Hope this helps.