New brakes, soft pedal, distribution blocks ?

There is nothing in a disk system that will "back the pistons out" except the deforming lip on the piston seal with relaxed pressure and that will withdraw the piston about .002-008 and that is by design. Thats all the pistons are going to move under pressure minus the caliper and line flex. You can disco and plug the rear circuit (one plug) and start bleeding from there. Should tell you what curcuit has the air. If you get the fronts rock hard, you know the rears will just need further bleeding to get a hard pedal. to bench bleed the MC just loop the 2 outputs back into their reservoirs under the fluid line and pump away until no bubbles are seen. Mighty vac on each bleeder will assist also, just make sure you dont run a reservoir dry.