On most duel reservoir master cylinders the reservoirs are two different sizes. The larger one is for the front disc brakes. Larger pistons in the front calipers than in the rear drum wheel cylinders require more fluid volume. When you hit the brakes the weight of the car is transfered forward so the front brakes do most of the work. If you have a master cylinder and both of the reservoirs are the same size and a little on the small size it is probably a drum/drum master cylinder.