the reasoning on the diffrent size lines front to rear is that disc brakes have more fluid demand than drum brakes. same as the master resevoir if you look the front has double the capacity of the rear. it takes more fluid pressure with the disc brakes to apply than the drum brakes in the back. in all actuality the engineers probably had 3/16 in the plans for the whole car but you have to remember that manufactures have a tendacy to use what they got. 1/4 line is cheaper than 3/16 so they would substitute to save a buck.
