what car is it on?

if its the escos in your sig and you've done away with the abs stuff but still left it plumbed in the answer is simple

on that system, the rear brakes get a set amount of pressure regardless of how hard you press on the brake pedal, so, a combination of mega stoppers on the front will make the rear end go light and, as you have found out, the rear is over braked by comparison and so locks up

put cheap pads on the rear and see what happens