Had a similar problem bleeding my brakes. I replaced the back slave cylinders and could not get anythingg to them. The pedal was still stiff and pushing and holding the button on the proportioning valve did not help either. I had to crack open the fronts and then was able to bleed the system. I would imaggine you will need to cack open the backs and pump away on the pedal to recenter the proportioning valve in order to bleed the fronts.
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