The master cylinder in action When you press the brake pedal, it pushes on the primary piston through a linkage.
Pressure builds in the cylinder and lines as the brake pedal is depressed further.
The pressure between the primary and secondary piston forces the secondary piston to compress the fluid in its circuit. If the brakes are operating properly, the pressure will be the same in both circuits. If there is a leak in one of the circuits, that circuit will not be able to maintain pressure. Figure 5 shows what happens when one of the circuits develops a leak. Now the master cylinder behaves as if it has only one piston.