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Hydraulic systems
See Figures 1, 2, and 3

When you step on the brake pedal, you expect the vehicle to stop. The brake pedal operates a hydraulic system that is used for two reasons. First, fluid under pressure can be carried to all parts of the vehicle by small hoses or metal lines without taking up a lot of room or causing routing problems. Second, the hydraulic fluid offers a great mechanical advantage-little foot pressure is required on the pedal, but a great deal of pressure is generated at the wheels. The brake pedal is linked to a piston in the brake master cylinder, which is filled with hydraulic brake fluid. The master cylinder consists of a cylinder containing a small piston and a fluid reservoir.

Modern master cylinders are actually two separate cylinders. Such a system is called a dual circuit, because the front cylinder is connected to the front brakes and the rear cylinder to the rear brakes. (Some vehicles are connected diagonally.) The two cylinders are actually separated, allowing for emergency stopping power should one part of the system fail.

The entire hydraulic system from the master cylinder to the wheels is full of hydraulic brake fluid. When the brake pedal is depressed, the pistons in the master cylinder are forced to move, exerting tremendous force on the fluid in the lines. The fluid has nowhere to go, and forces the wheel cylinder pistons (drum brakes) or caliper pistons (disc brakes) to exert pressure on the brake shoes or pads. The friction between the brake shoe and wheel drum or the brake pad and rotor (disc) slows the vehicle and eventually stops it.

Also attached to the brake pedal is a switch that lights the brake lights as the pedal is depressed. The lights stay on until the brake pedal is released and returns to its normal position.

Each wheel cylinder in a drum brake system contains two pistons, one at either end, which push outward in opposite directions. In disc brake systems, the wheel cylinders are part of the caliper (there can be as many as four or as few as one). Whether disc or drum type, all pistons use some type of rubber seal to prevent leakage around the piston, and a rubber dust boot seals the outer ends of the wheel cylinders against dirt and moisture.

When the brake pedal is released, a spring pushes the master cylinder pistons back to their normal positions. Check valves in the master cylinder piston allow fluid to flow toward the wheel cylinders or calipers as the piston returns. Then as the brake shoe return springs pull the brake shoes back to the released position, excess fluid returns to the master cylinder through compensating ports, which have been uncovered as the pistons move back. Any fluid that has leaked from the system will also be replaced through the compensating ports.

Figure 1 Typical components found on a front disc/rear drum brake system.
Typical components found on a front disc/rear drum brake system.

Figure 2 Typical master cylinder. Since 1967, all master cylinders are of the dual circuit type.
 Typical master cylinder. Since 1967, all master cylinders are of the dual circuit type.

Figure 3 Exploded view of a typical wheel cylinder used on drum brakes.
Exploded view of a typical wheel cylinder used on drum brakes.

All dual circuit brake systems use a switch to activate a light, warning of brake failure. The switch is located in a valve mounted near the master cylinder. A piston in the valve receives pressure on each end from the front and rear brake circuits. When the pressures are balanced, the piston remains stationary, but when one circuit has a leak, greater pressure during the application of the brakes will force the piston to one side or the other, closing the switch and activating the warning light. The light can also be activated by the ignition switch during engine starting or by the parking brake.

Front disc, rear drum brake systems also have a metering valve to prevent the front disc brakes from engaging before the rear brakes have contacted the drums. This ensures that the front brakes will not normally be used alone to stop the vehicle. A proportioning valve is also used to limit pressure to the rear brakes to prevent rear wheel lock-up during hard braking.

Brake fluid
See Figure 4

Clean, high quality brake fluid is essential to the safe and proper operation of the brake system. You should always buy the highest quality brake fluid that is available. If the brake fluid becomes contaminated, drain and flush the system, then refill the master cylinder with new fluid. Never reuse any brake fluid. Any brake fluid that is removed from the system should be discarded.

At first, vehicles used mechanically actuated brakes. Simple and easy, the system worked okay until vehicles started going more than 10 miles per hour on a regular basis. At this point, the idea for hydraulically actuated brakes came up.

A liquid, for all practical purposes, can't be compressed. So, if you fill a sealed system with liquid and try to compress it, say with a master cylinder, the liquid exerts equal force on all other parts of the system, for instance calipers and wheel cylinders. Get the idea?

Figure 4 When filling the master cylinder, use only clean, fresh fluid specified for your vehicle.
When filling the master cylinder, use only clean, fresh fluid specified for your vehicle.

Vehicle manufacturers recognized the need for a fluid that resisted high temperatures, had lubricating capabilities, had a low freezing point and resisted corrosion. Almost all vehicles on the road today use brake fluid designated DOT 3 or DOT 4. DOT stands for Department of Transportation. The DOT established the standards by which brake fluid, among other things, is regulated.

Some vehicles have been built using a silicone-based (DOT 5) brake fluid, but these are few and far between. DOT 3 and 4 fluids are petroleum-based liquids. Silicone fluids are, of course, not petroleum-based and are completely incompatible with other types and may cause damage to the rubber seals if added to systems that are not designed for silicone fluid. There are 2 chief advantages to silicone-based brake fluid. For one thing, it has a superior ability to withstand heat. And for another, it does not share the petroleum-based fluid's tendency to absorb moisture. However, petroleum based fluids are perfectly able to withstand the heat generated by just about all modern vehicles. If you maintain the DOT 3 or 4 fluid in your brake system through periodic changes, and keep the system sealed to protect it from dirt or moisture, silicone based fluids are unnecessary.

The best rule of thumb with brake fluid is to use the fluid recommended by your owner's manual. Changing the fluid from DOT 3 to DOT 4 is often allowed, but do not change from DOT 3 or 4 to DOT 5.

Brake fluid is a specialized liquid and should never be mixed with any other type of fluid, such as mineral oil. Also, brake fluid has the ability to absorb moisture from the air, so, it can become contaminated simply by age. Over the years, you'll be removing the master cylinder cap or disconnecting brake lines. During the time that the system is open, the brake fluid will absorb small amounts of moisture, thereby reducing its effectiveness. Brake fluid contaminated with moisture will cause rust in the system as well as losing its ability to stand up to heat.

Therefore, it is recommended by many vehicle manufacturers and most professionals that the brake fluid system be flushed and refilled every 2 years. This is especially true on vehicles with ABS systems.

When adding brake fluid to your vehicle's brake system, use brake fluid that is fresh and kept in a small, sealed container. If your brake fluid jar has been sitting around for a while, get new stuff. It's not expensive and is critical to the performance of the braking system. Don't forget, use only approved DOT 3 or 4 fluid. If you're in doubt about the fluid recommendation for your vehicle, check your owner's manual.

Used fluids such as brake fluid are hazardous wastes and must be disposed of properly. Before draining any fluids, consult with your local authorities; in many areas waste oil, etc. is being accepted as a part of recycling programs. A number of service stations and auto parts stores are also accepting waste fluids for recycling.

Be sure of the recycling center's policies before draining any fluids, as many will not accept different fluids that have been mixed together.

A final note...brake fluid should be handled with care. Brake fluid is a nasty and poisonous substance. Keep it out of your eyes and off your skin. Also, remember that it is an excellent paint remover. If you don't care for your personal safety, think of your vehicle; if brake fluid gets on your vehicle's paint, wipe if off immediately and rinse the area with water. It is probably not a bad idea to also clean the area with a gentle household cleaner or car wash detergent

Friction materials
See Figure 5

Brake shoes and pads are constructed in a similar manner. The pad or shoe is composed of a metal backing plate and a friction lining. The lining is either bonded (glued) to the metal, or riveted. Generally, riveted linings provide superior performance, but good quality bonded linings are perfectly adequate.

Friction materials will vary between manufacturers and type of pad and the material compound may be referred to as: asbestos, organic, semi-metallic, metallic. The difference between these compounds lies in the types and percentages of friction materials used, material binders and performance modifiers.

Asbestos was used in friction lining compounds for many years because of its ability to withstand heat, but became less popular as health concerns arose. It may still be found in different types of pads and, unless you personally replaced the last set of brakes on your vehicle, for safety reasons you should assume that a compound containing asbestos may have been used.

Generally speaking, organic and non-metallic asbestos compound brakes are quiet, easy on rotors and provide good feel. But this comes at the expense of high temperature operation, so they may not be your best choice for heavy duty use or mountain driving. In most cases, these linings will wear somewhat faster than metallic compound pads, so you will usually replace them more often. But, when using these pads, rotors tend to last longer.

Figure 5 When purchasing brake pads (and shoes) remember that in the end, they are the only things stopping your vehicle.
When purchasing brake pads (and shoes) remember that in the end, they are the only things stopping your vehicle.

Semi-metallic or metallic compound brake linings will vary in performance based on the metallic contents of the compound. Again, generally speaking, the higher the metallic content, the better the friction material will resist heat. This makes them more appropriate for heavy duty applications, but at the expense of braking performance before the pad reaches operating temperature. The first few applications on a cold morning may not give strong braking. Also, metallics and semi-metallics are more likely to squeal. In most cases, metallic compounds last longer than non-metallic pads, but they tend to cause more wear on the rotors. If you use metallic pads, expect to replace the rotors more often.

When deciding what type of brake lining is right for you, keep in mind that today's modern cars have brake materials which are matched to the expected vehicle's performance capabilities. Changing the material from OEM specification could adversely affect brake feel or responsiveness. Before changing the brake materials, talk to your dealer or parts supplier to help decide what is most appropriate for your application. Remember that heavy use applications such as towing , stop and go driving, driving down mountain roads, and racing may require a change to a higher performance material.

Some more exotic materials are also used in brake linings, among which are Kevlar® and carbon compounds. These materials have the capability of extremely good performance for towing, mountain driving or racing. Wear characteristics can be similar to either the metallic or the non-metallic linings, depending on the product you buy. Most race applications tend to wear like metallic linings, while many of the street applications are more like the non-metallics.

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