Before 1967 cars and light trucks were fitted with a master cylinder that only had one piston and one hydraulic circuit to operate the brakes on all four wheels. If there was a leak some where in the system it affected the brakes on all four wheels. To add redundancy engineers added a mechanical emergency brake that was activated by hand or foot and connected to the brakes by cables to stop the vehicle during brake failure. Today the emergency brake is called a parking brake.
In 1967 a new law required automotive manufacturers to use tandem master cylinders to operate the brakes. A tandem master cylinder splits the hydraulic circuit into two. In case one circuit fails you still have the other circuit to stop the car or light truck. One hydraulic circuit can operate the front wheels and the other circuit the rear wheels, or one hydraulic circuit can operate the left front wheel and right rear wheel and the other circuit the right front wheel and left rear wheel. As a result of using dual or tandem master cylinders the emergency brake is now called a parking brake.
Photo of an old vehicle with a single master cylinder
