Vehicle braking systems operate by applying a friction material, that moves on a fixed axis, to a rotating mass. This effort converts kinetic energy into heat bringing the vehicle to a halt. On vehicles there are two kinds of brakes fitted; the drum brakes and the disc brakes. The drum brake uses internal expanding brake shoes that push out on to the internal face of the brake drum, and disc brakes have externally fitted brake pads that sandwich the brake disc when pressure is applied. Most popular cars have disc brakes fitted the front axle with drum brakes fitted to the rear, with high specification and sports models having discs brakes all round. More recently many manufacturers are fitting disc brakes as standard to all but the very basic models.
During braking effort very high temperatures are generated and when friction material overheats drivers will experience a condition known a brake fade. When brake fade is experienced the driver will need to apply greater pressure to the foot brake and the vehicle will take longer to stop. As brake discs are exposed to the air rushing over them, heat is dissipated, temperature is controlled and the disc brakes are not affected by brake fade. Brake drums, however, are enclosed, generate heat internally, are prone to overheating and brake fade, typically when braking from high speeds or travelling in mountainous regions. Brake discs are also more efficient at self adjusting as the brakes wear, maintaining proportional braking effect to braking effort.
When the brake pedal is depressed, pressure is applied to a push-rod forcing the piston inside the brake master cylinder forward. This forward movement pushes the hydraulic fluid out of the master cylinder through the thin brake pipes. This hydraulic pressure then acts on the pistons of the brake callipers or wheel cylinders and it is the movement of the pistons in the callipers or the wheel cylinders that force the friction material in to contact with the discs or drums. When a vehicle starts to slow down, inertial forces cause the vehicle weight to move to the front axle, so greater braking effort is needed on the front wheels. This is typically a 60 - 40 % split, which is governed by a pressure regulating valve, which also diverts hydraulic pressure to front brakes once a set pressure is reached at the rear brakes.
In order for a car to stop, a great deal of pressure needs to be applied to the brake cylinders. For driver comfort and safety modern cars use assisted braking. Modern cars are fitted with a direct acting servo, to which the brake master cylinder is bolted. The servo provides assistance to the braking effort by exerting addition force to the master cylinder push rod when the brake pedal is depressed. The servo is made up from a large chamber divided by a diaphragm and spring, with one side of the chamber connected via a pipe and non return valve to the inlet manifold with the other side of the chamber being able to vent to atmosphere. The suction from inlet manifold creates a vacuum within the chamber and when the brake is not applied vacuum is present in both sides of the chamber. When the brakes are depressed air is allowed to enter one side of the chamber, and it the pressure difference between atmospheric pressure and the vacuum in the chambers that causes the servo piston to move, pushing the rod in the master cylinder forward.
Brake callipers are used with disc brakes. They typically have two opposed pistons enclosed in a aluminium housing. The housing has internal drillings connecting the two piston chambers and each piston is fitted with a seal which prevents fluid leaks. The housing will also have a bleed valve from which air can be removed from the hydraulic system. When the brake pedal is depressed hydraulic pressure in the piston chamber acts on the piston, forcing it out, which in turn clamps the brake pads against the brake disc.
When any repairs or maintenance are carried out on a braking system, safety must be the primary concern. Defective or poorly maintained brakes can lead to accidents, and injury to the driver and other road users. Routine maintenance must include an inspection of the hand brake mechanism, ensuring that the cables and ratchet assembly move easily and are free from damage. Appropriate adjustment to the handbrake must be made and all linkages lubricated. The wheels and drums need to be removed so that the brake linings can be inspected for wear, contamination, de-bonding and cracking. The pads need to be removed from the calipers and the calliper sliders lubricated. The discs and drums need to be measured to ensure they are not below manufacturer's tolerance and checked they are free from excessive corrosion. Before the brakes are re-assembled they need to cleaned with an appropriate fluid and the non friction surfaces lubricated with a high temperature grease. The hydraulic pipes must be checked for excessive corrosion, cracks, splits and leaks and the hydraulic assemblies checked for leaks and operation, as calipers and wheel cylinders frequently become seized. Finally a road test is required to test the efficiency of the brakes, and that the vehicle comes to a halt in a straight line without pulling vigorously to one side.