The hydraulic brake circuit has a fluid-filled master cylinder and a slave cylinder connected by a pipe.
When you step on the brake pedal, it depresses the piston in the master cylinder, forcing fluid to flow along the pipe.
The fluid travels on each wheel to the slave cylinder and fills them, forcing the piston to apply a brake outward.
Fluid pressure distributes itself evenly around the system.
The combined surface "push" area of all slave pistons is much larger than the surface of the piston in the master cylinder.
Consequently, the master piston has to travel several inches to move the slave pistons the fraction of an inch it takes to apply the brakes.
This arrangement allows great force to be exerted by the brakes, in the same way that a long-handled lever can easily lift a heavy object a short distance.
Most modern cars are equipped with dual hydraulic circuits and two master cylinders are connected in series to prevent malfunction.
Sometimes one circuit is used for the front brake, one circuit is used for the rear brake; or each circuit is used for one of the front and rear brakes; or one circuit can only work four brakes and the other brake.
Under heavy braking, so much weight may come off the rear wheels that they lock, possibly causing a dangerous skid.
For this reason, the rear brakes are deliberately made less powerful than the front.
Most cars now also have a load-sensitive pressure limiting valve. When severe braking raises hydraulic pressure to a level that can cause the rear brake to lock, it closes and prevents fluid from flowing further to them.
Advanced cars may even have complex anti-lock systems that sense in various ways how the car is decelerating and whether any wheels are locking.
Such systems apply and release the brakes in rapid succession to stop them locking.