The car master cylinder is a component that converts non-hydraulic pressure into hydraulic one. The device controls slave cylinders located at the other end of the hydraulic system. Whenever you press the brake pedal, it pushes the primary piston through a linkage; more pressure is building on the cylinder and lines as the pedal is depressed further. The pressure between the primary and secondary piston forces the secondary piston to compress the fluid in its circuit. The pressure in both the circuits will be the same if the brakes are working properly.
In case any of the circuits is leaking, the leaked circuit won’t be able to maintain the pressure. When the first circuit leaks, the pressure on both cylinders is lost. This leads primary cylinder to contact the secondary and master cylinder starts behaving like if it has only one piston. However, the second circuit will work properly, but the driver has to press the pedal further to activate this. Since only two wheels have the pressure, the braking power of the vehicle will be reduced.