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.