Just like today, the performance options on older vehicles were a profit centre for the factory and dealer so that they supplied the cheapest, worst-performing braking systems as standard equipment. You could then pay extra for performance upgrades like better brakes. However, most buyers never tested the brakes under real-world emergency braking situations and just took the standard brakes that came with the car. It wasn't as bad in the old days when everyone had bad brakes but modern vehicles have far superior brakes compared to the OEM premium brakes in the old days. The net result is that a sudden stops at highway speeds today can mean your older car is in danger of rear-ending other cars, even if you've got great reaction time and maintain a good space cushion while you're driving. Braking improvements were mandated by the US Federal Government with NHTSA Standard 105 -68 (effective on January 1, 1968) and 105-75 (effective January 1, 1976). See NHTSA Report Number DOT HS 806 359.
Dual Master Cylinder
With the single chamber master cylinders, a hydraulic failure at one wheel means a failure in the entire system. Although the emergency (parking) brake provides for backup mechanical braking, it is totally inadequate to stop a moving vehicle. A dual master cylinder splits up the braking system so that a hydraulic failure at any one wheel still leaves two functioning brakes. The easiest way to upgrade is to adapt a newer master cylinder from the same brand (and preferably model) of vehicle to an older model.
Disc Brakes were mandated by the US federal government in the early 1975. Prior to that, drum brakes were usually standard equipment and base model vehicles usually got the smallest drums. A more involved upgrade is to change your drum brakes to disc brakes and it is best to take a complete system from a newer model of your car. In some cases, the bolt pattern is different (like on Chrysler A-Body cars) so that you also need to change the rear axle as well so that all wheels have the same bolt-pattern. Normally, a disc brake upgrade also includes a dual master cylinder. For more advice about this type of upgrade, please look for an internet forum specific to your vehicle.