553 Brake Systems Technician Handbook Hydraulics A hydraulic brake system is a closed system filled with liquid. Because liquids cannot be compressed, when pressure is applied to the master cylinder piston that same amount of pressure is instantly applied to the entire system. So if 100 psi of force is created at the master cylinder, then 100 psi of force will be seen at each wheel. If there were just one wheel cylinder at the other end of the hydraulic system and it had the same diameter as the master cylinder, that wheel cylinder would travel the same distance as the master cylinder and exert the exact same amount of pressure.
But brake systems need much more pressure than the driver can apply to stop the vehicle. So they use ‘ force multiplication’ to increase the pressure at each wheel. Drum brakes have a larger cylinder volume than the master cylinder, so when pressure is applied the brake travels a shorter distance but applies a greater pressure. Disc brakes are smaller and have less contact with the wheel than drum brakes, so they require even more pressure to create the same braking force. Disc brake have an even larger cylinder volume so they can take greater advantage of force multiplication. Hydraulic systems only work because liquids cannot be compressed.
If air is introduced into the system, the gas will compress under pressure and the system won ’ t be able to apply full pressure to the wheels. Technical Training 5