One of the first automakers to introduce dual circuit brakes was American Motors in the early 1960s for the Rambler line. The system had dual channels in an x-pattern, so if one brake line failed it still had at least one front and rear brake working to stop the vehicles. Other automakers had a single hydraulic circuit for the front and rear.
Ramblers also had a brake pressure differential switch. This switch had to have an equal force pushing on both sides of a sliding valve. If one side had a leak, the hydraulic force would push the valve over and seal the leaking circuit.
The dual master cylinder was mandated for the 1967 model year. Rambler was America’s Volvo in the 1960s. While other automakers like GM and Chrysler were selling power and luxury, Rambler was preaching safety and fuel economy. Rambler’s tagline was their cars were for the “human race.”