2 Factory dual braking systems were federally mandated beginning with the ’67 model year. In a dual braking system, braking is divided into two systems—one for the front brakes and a second one for the rear. The master cylinder’s forward reservoir and cylinder operates the rear brakes and the aft reservoir and cylinder work the fronts. Disc brake master cylinders have a larger rear reservoir to operate the front disc brakes, which need more fluid capacity. With four-wheel drum brakes, you will see a dual reservoir master cylinder with equal size reservoirs. The illustration here shows a disc brake master cylinder.