Not enough vacuum will cause a hard pedal not a low one. Assuming all the air is truly out of the system, I would adjust up the rear brakes first. Then, check if there is an adjustment on the rod between the pedal and the mc. Next i would isolate both calipers and rear wheel cyl from the master, Then see if your pedal is solid. if it is, you have a problem at one of the wheels. isolate them individually to determine which wheel your problem is at.
