Car components : Braking System

Conventional braking system

Most braking systems on passenger cars are hydraulically operated.

• The force of fluid under pressure operates your brakes. Pressure is applied to the fluid by the brake master cylinder with assistance from the brake booster.
• The master cylinder is a pump containing pistons and a reservoir of fluid.
• When pressure is applied to the pistons by the brake pedal, fluid is pushed out under pressure to the master cylinder outlets, which are connected to the brake lines.
• The fluid travels through the lines to the wheel cylinders or calipers. They in turn apply pressure to the brake pads or linings.
• The brake pads and linings are forced against the brake discs and drums and friction causes the discs and drums to slow down and stop.
• Some cars have drum brakes only, some have disc brakes only and some have a combination of the two.
• The bag, made of nylon material, is stored in the steering wheel or the dashboard under a cover and inflates and deflates rapidly during specific types of collision.
• Some cars have drum brakes only, some have disc brakes only and some have a combination of the two.
• The latter would have disc brakes on the front and drum brakes on the rear of the car.

Master cylinder, brake booster and brake lines

• Master cylinder. The master cylinder is usually mounted to the booster, which is bolted to the firewall. A rod runs from the brake pedal through the booster and acts on the master cylinder pistons.
• Brake booster. The booster increases the force exerted on the master cylinder pistons by the brake pedal. It does this by applying vacuum, supplied by a hose from the inlet manifold, to one side of a diaphragm and atmospheric pressure to the other.
• Brake lines. The brake lines from the master cylinder are metal and carry the fluid to the calipers and wheel cylinders. Because wheels on the front of the vehicle must turn when cornering and all the wheels must move up and down over bumps, the final part of the brake line is made from a flexible material. The flexible brake lines are prone to perishing and cracking and should be checked regularly.

Disc brakes

The components and details of the disc braking system are:

• Disc rotor. The rotor is a metal disc attached to a hub, which is in turn connected to the wheel by the wheel studs and nuts. The rotor surface should be smooth and of a specified thickness. Because of the heat generated while braking, the manufacturer recommends a minimum thickness for brake disc rotors. When brake rotors are scored or grooved, they can be machined smooth again providing the minimum thickness is not exceeded.
• Hydraulic caliper. The hydraulic caliper is a cylinder with a piston or pistons sliding inside. When the brake pedal is pressed, fluid is sent under pressure in to the caliper, which pushes the piston forward. The movement of the piston applies pressure to the brake friction pads, which contact the rotor and slow its spinning motion.
• Brake friction pads. The friction pads are made from a combination of materials. These parts need to be replaced occasionally.

Drum brakes

The main components of the drum brake system are:

• Brake drum.
• Brake shoes.
• Wheel cylinder. When the pedal is depressed fluid is pumped into the wheel cylinder, which forces the pistons apart. They in turn push the brake shoes against the spinning brake drum, slowing it down.
• Return springs. The return springs pull the brake shoes back into their original position when the brake pedal is released.
• Handbrake mechanism. The handbrake mechanism is attached to the shoes and does the same job as the wheel cylinder, the only difference is that this time the shoes are moved by mechanical force rather than hydraulic pressure. The handbrake mechanism is connected to the handbrake lever inside the car by a single or multiple cables.
• Adjuster. The most common type of break adjuster in use today is the threaded type, which sits between the shoes and is turned by the handbrake lever. The job of the adjuster is to maintain a minimum clearance between the shoes and drum to optimise brake pedal travel. This type of mechanism is self-adjusting and requires little maintenence.
• Common problems. The brake pads and shoes contain a friction material, this material wears down with use and should be checked in accordance with the manufacturer's instructions. Disc rotors and brake drums become scored and the surface becomes uneven with wear. Occasionally they will need to be machined, if within tolerances, to ensure efficient brake performance. Brake fluid is hydroscopic in nature and because of this tendency to absord water and contaminants it should be flushed out and renewed in accordance with the manufacturer's service schedule. The entire brake system should be checked regularly. Metal lines should be examined for corrosion and fracture. Flexible hoses should be checked for cracking and fraying. Wheel cylinders and calipers should be checked for leaks and seizing.

Anti lock braking system (ABS)

The most common reaction of a driver in an emergency situation is to slam their foot on the brake pedal, this is a normal reaction, but one that can make a bad situation worse.

• Conventional braking systems are fine in most situations, but when it comes to an emergency braking situation in wet and slippery conditions ABS has the edge.
• ABS stops the wheels from locking up and gives the driver more control over direction.
• In most situations a car with ABS will stop in a shorter distance than a car without.
• ABS detects when a wheel is about to lock up and restricts the brake line pressure to that wheel, so the wheel remains turning and the driver stays in control.
• Under normal braking conditions, ABS does virtually nothing except monitor the wheel speed.

Components of the ABS system

• Wheel speed sensors. Wheel speed sensors tell the control unit when a wheel is about to lock up. They are located in the hub beside the wheels and sometimes in the differential. There are two systems in common use, one uses three sensors, one for each front wheel with a third sensor in the rear differential and the other uses four sensors, one for each wheel.
• Solenoid valves. Solenoid valves are in the brake lines going to each wheel and are controlled by the ABS control unit. When the wheel sensor detects a wheel starting to lock up, the control unit shuts off the solenoid valve for that wheel and reduces the pressure in the line by bleeding the fluid back to the master cylinder.
• Pump. The pump applies pressure to the brake line.
• ABS control unit. The ABS control unit is a computer, which monitors the feedback from the wheel sensors and operates the solenoid valves and pump. An ABS warning light in the instrument cluster will come on when the ignition is turned to the on position. The control unit will go through a self-diagnosis and if all the components in the system are operating within given parameters, the light goes out. If the light comes on while driving, take the car to a specialist for diagnosis. If the ABS ceases to operate, the conventional brake system will still work and there will be no noticeable difference under normal braking conditions.