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The fronts have no air , so the pedal can't go to the floor.. it can only go as far as it takes to fully engage the front brakes . If you open a front bleeder , only then it will go to floor.
If the situation was reversed [ air in front lines and none in rear], same thing would happen.
There are two brake systems , but they are hooked in Tandem at the plunger in the Master.
As others have mentioned , we would like to know for sure that you have the rear res full. It is very common to overlook this as many do not realise there are two res. One for the front , one for the back..
Read Sokoloff again .