A typical braking system as shown, consists of disc brakes at the front and drum brakes on the rear connected by a series of metal pipes and hoses. Most new cars have disc brakes all round. When you step on the brake pedal, you are actually pushing against a plunger in the master cylinder which forces brake fluid through the connecting pipes to a braking unit at each wheel.
On a disc brake you have a rotating disc attached to the wheel which runs through a caliper with brake pads attached. When the foot pedal is depressed fluid pushes through the system and squeezes the brake pads onto the discs. The operation is very much the same as on a bicycle except that the calipers are operated hydraulically rather than via a cable. Drum brakes have a wheel cylinder where the hydraulic fluid pushes a small piston outwards in both directions which in turn pushes the brake shoes outwards against the drum in which they sit.
On both types of brake it is important to change the pads/shoes before they wear out. BEST Autocentres recommend that brake pads are changed when the friction
material has worn down to 3mm and brake shoes are changed at
2mm. It is vital that brakes are replaced evenly across an axle, if
only one side is replaced an imbalance will occur and the car
will pull to one side on braking.