A hydraulic system (Figure 46-4) uses a brake fluid to
transfer pressure from the brake pedal to the pads or
shoes. This transfer of pressure is reliable and consistent
because liquids are not compressible. That is, pressure
applied to a liquid in a closed system is transmitted by
that liquid equally to every other part of that system.
Apply a force of 5 pounds (35 kPa) per square inch (psi)
through the master cylinder and you can measure 5 psi
(35 kPa) anywhere in the lines and at each wheel where
the brakes operate.