Many people are not technically knowledgeable about vehicle brake systems, often because they don’t spend time in the driveway repairing them! For those that are involved in the investigation of vehicle accidents, the primer below will hopefully provide an easy to understand background on automobile brakes. The intent is not to tell you what things can go wrong, but to provide an understanding of how these systems work.
AUTOMOTIVE HYDRAULIC BRAKES - A PRIMER
For the most part, vehicles now come equipped with hydraulic brake systems that include disc brakes on the front and drum brakes on the rear, although 4 wheel disc brakes are becoming more prevalent. Brake systems include a combination of the disc and drum type brakes, a dual circuit master cylinder, some form of brake booster (usually vacuum style), hydraulic brake lines, and in most cases, an anti-lock brake system (ABS). See Figure 1.
ABS systems do not change the basic design of the automobile brake systems but add an additional control system to the brakes in order to maximize braking capability and steerability during an aggressive braking situation. ABS is not discussed in this article.
When the driver applies the brake by pushing on the pedal, a pushrod from the brake pedal pushes on the vacuum booster diaphragm which the amplifies the force produced at the brake pedal, giving the vehicle its ‘power brakes’. The pushrod from the booster subsequently pushes the piston in the dual circuit master cylinder. As shown in Figure 1, the brake fluid pressure is transferred from the master cylinder to the front and rear brakes.
Brake systems are designed with a proportioning and pressure regulating valve as specified by each manufacturer. The more aggressive the braking, the more of the vehicle's weight is transferred to the front wheels which tends to cause the rear to lift and the front to dive. The valves are designed to direct more pressure to the front and less pressure to the rear in order to reduce the chance of premature lockup at the rear wheels.
The brake system is usually split into two circuits, which may be of two types. The first is a diagonal split, where one side of the dual circuit master cylinder controls the right front and left rear brakes (‘right circuit’) and the other controls the left front and right rear brakes (‘left circuit’). The second type of split is from front to back with one side of the dual circuit master cylinder controlling the front brakes (‘front circuit’) and the other controlling the rear brakes (‘rear circuit’).
The purpose of having 2 circuits is to ensure that, if a failure occurs on one circuit, then the vehicle still has 50% braking capability from the second circuit. The reservoir on the master cylinder is also split into 2 sections, one to feed each circuit. Therefore, if there is no fluid on one side of the brake fluid reservoir, the other circuit is not affected. There is a float in the reservoir that measures the height of available fluid and which is designed to illuminate a brake warning light on the driver’s dashboard before the fluid level inside either side of the split reservoir is too low for normal operation.
When a driver brakes, fluid is forced from the master cylinder through the brake lines into the wheel cylinders of each rear drum brake. Pistons in the cylinder push outward onto the brake shoes. This causes the friction linings of the brake shoes to be pressed outward against the drum which is attached to the wheel. The friction between the stationary shoes and the revolving drums causes the drums to slow and stop the rear wheels. Brake drums are made of iron and have a machined surface on the inside where the shoes make contact. The parking brake (emergency brake) cable is connected to the rear brakes.
The wheel cylinder on a drum brake consists of a cylinder that has two pistons, one on each side. Each piston has a rubber seal and a shaft that connects the piston to a brake shoe. When brake pressure is applied, the pistons are forced apart and push both brake shoes into contact with the drum. Wheel cylinders are replaced if they show signs of leaking as leakage could cause loss of pressure on that circuit of the brake system or result in brake fluid contaminated brake shoes and therefore could increase braking distance.
When the brake pedal is pushed on disc brakes, brake fluid from the master cylinder forces the caliper piston outward to compress the brake pads against the rotors. The friction between the stationary pads and the revolving rotor causes the rotors and wheel to slow and stop. Disc brakes are a lot like the brakes on a bicycle that have a caliper and squeeze the brake pads against the wheel. On an automobile disc brake, the brake pads squeeze the rotor instead of the bicycle wheel, and the force is transmitted hydraulically instead of through a cable from the handlebars.
Because a disc brake assembly can absorb more heat than a drum brake assembly and are generally more efficient technically, most vehicle manufacturers use disc brakes for the front wheels with more and more using them at all four wheels.
Note: many of the images in this paper were provided courtesy of Hunter Engineering (www.hunter.com), a leading manufacturer of vehicle service equipment