Hmm this is a funny one lol..The hopping up and down on the pedal is that not the ABS kicking in? but then again why are you locking up if that was the case? as you shouldnt with ABS have you tryed takeing the brake master cylinder stopper off again to see if its any better? i cant see it been the Dot 4 causeing it to lock up either as its locking up befor the fluid is getting hot!