You say you adjusted the bias until the rear stopped locking up. Am I to assume that the fronts now lock first? If so, where is the balance bar set in relationship to the master cylinders and brake pedal? Is it roughly centered or....... measurements will be handy. Make sure that the bar is perpendicular to the pushrods when the pedal is fully depressed, that the master cylinders can fully return and that there is no binding throughout the travel.