The Brake System - Maintenance
Brake System Topics
  1. The ABS Light Comes On - Is It Still Safe to Drive?
  2. Bleeding the Brakes
  3. Brake Calipers
  4. What Is Included in a Complete Brake Job?
  5. About Brake Lining Replacement
  6. Brake System Symptoms Checklist
  7. Brake Cleaning Solvents
  8. Rotor Drum Turning Issues
  9. The Difference Between Four Wheel ABS and Rear Wheel ABS
  10. Why It Is Important to Periodically Replace Brake Fluid
  11. Rotor Surface Finishes
  12. Turning New Rotors or Drums

The ABS Light Comes On - Is It Still Safe to Drive?

What is ABS?
ABS stands for Anti-lock Brake System. The ABS was designed to prevent brakes from locking during panic stops. Often, during panic stops, a driver applies too much pressure to the brake pedal and the brakes lock-up, causing the wheels to skid and a loss of vehicle control.

The ABS uses a computer and sensors to monitor wheel rotation speed and hydraulic brake pressure. Using that information, the computer determines and controls the optimum amount of hydraulic pulses sent to the brake calipers to maximize the brake pressure without locking-up.

It Is Still Safe to Drive, But ...

ABS Light On - Fault Detected?
An ABS warning light means the ABS system has been deactivated because of a self-diagnosed fault. Normal braking should remain. The vehicle should be safe to drive, but the driver does not have the ABS system to help him when panic braking on a wet or slick surface. This distinction is important because the driver will have to use less braking pressure to prevent the brakes from locking up, perhaps resulting in loss of control.

ABS Light On - Loss of Power Assist?
An ABS warning light may also indicate a loss of power-assist if the vehicle has an ABS system relying on an electric pump and pressure accumulator pump rather than a conventional vacuum booster.

The car will still brake, but will not have the usual power assist. This could create an unsafe situation for drivers who have difficulty braking without power assist.

ABS Light On - Faulty Wheel Speed Sensor?
An anti-lock warning lamp that comes on when the car starts moving, or anti-lock braking operation or valve cycling that occurs during normal stops on dry pavement, often indicates a problem with one of the wheel speed sensors.

ABS Light On - Other Possibilities.
When both brake and antilock warning lights are on, and there is a lack of normal power assist, the pump may be inoperative on applications that have an integral ABS system.

When both warning lamps are on and power assist is present, it may indicate a low brake fluid level or loss of hydraulic pressure in one of the brake circuits.

ABS is essentially an add-on to the existing brake system. It only comes into play when traction conditions are marginal or during sudden panic stops. The rest of the time, it has no effect on normal driving or braking.

If brakes are pulling or grabbing during normal braking, it is not an ABS problem. The vehicle has a conventional brake problem needing attention.

A brake warning lamp (not ABS lamp) that remains on or comes on while driving, usually signals a problem with the hydraulic system, not the ABS system. There may be a fluid leak or loss of pressure, either of which pose a danger to safe braking. The cause of the brake warning light should be investigated immediately.

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Bleeding the Brakes

What Is Bleeding the Brakes?
Bleeding is a brake service procedure that involves purging air from the master cylinder, brake lines, calipers, and wheel cylinders.

If there are air bubbles in the fluid, they will compress when the brakes are applied, causing either a low or soft pedal. Bleeding gets the air out, leaving only non-compressible brake fluid.

How Does Air Get Into the Brake System?
Air can enter the brake system in a variety of ways: when the system is opened for repairs, if the master cylinder reservoir gets too low, a brake cylinder gets a crack, a loose brake line fitting, or road damage.

Methods for Removing the Air
To remove the air, bleeder screws on the calipers and/or wheel cylinders are opened one at a time. Old fluid, along with any air, is then drained, pumped, pushed or siphoned out while fresh fluid is added to the master cylinder reservoir.

There are several common methods in use:

The Bleeding Sequence
To get all the air out, brakes must be bled in the proper sequence. Depending on how the hydraulics are split (front/rear or diagonally), the usual sequence is to bleed the wheels furthest from the master cylinder, then the closest wheels.

On most rear-wheel drive vehicles, the recommended sequence is RR, LR, RF, LF. On front-wheel drive cars with diagonally split brake systems, the sequence is RR, LF, LR, RF. If the master cylinder is being replaced, it may have to be bench bled before being installed.

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Brake Calipers

When to Rebuild or Replace?
Brake assemblyAny caliper that is leaking, has worn or damaged seals, or is causing brake pads to wear unevenly, needs to be rebuilt or replaced, but so do many calipers that appear to be trouble-free.

After three or four years of service, most caliper bores and steel pistons have visible corrosion and pitting. As the surface of the piston becomes rough, it starts to wear the piston seal. Every time the brakes are applied, the roughness on the piston scrapes back and forth across the seal. Eventually, the seal will fail and the caliper will leak.

We inspect the seal when the brakes are relined and replace the seal if indicated. In our experience, replacing the seals at the second brake relining or at 100,000 miles is adequate.

In a sliding caliper, only one side of the caliper has an apply piston. The caliper moves in relation to the rotor and is held in a frame rigidly attached to the steering knuckle.

As linings wear, the piston gradually moves further out in the caliper bore as the pads wear. When the piston is shoved back in to accommodate new thicker pads, any dirt or corrosion on the piston will be forced under the seal and accelerate seal wear.

Partial brake assembly - exploded viewAnother reason for rebuilding calipers is because rubber piston seals deteriorate with age. A piston seal performs a two-fold function; it seals the piston so hydraulic pressure can apply the brakes, and it helps retract the piston when the brakes are released.

As the piston is pushed out by the brake fluid, a square-cut seal twists slightly. This helps pull the piston back when the pressure is released, allowing pads to move away from the rotor more easily for reduced brake drag and improved pad wear and fuel economy.

Heat ages the seal. Over time, it loses elasticity and becomes brittle. This reduces its ability to deform and pull the piston back.

Brake calipers - cross sectionA neglected caliper can become a dragging caliper, causing increased pad wear, fuel consumption, and possibly a steering pull.

Rebuilding a caliper usually costs less than replacing it with a remanufactured or new unit, but it does involve extra time and effort.

Many professionals prefer the convenience of replacing old calipers with rebuilt units rather than rebuilding the calipers themselves. If a caliper can't be rebuilt because of damage or severe wear, replacement is the only option.

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What Is Included in a Complete Brake Job?

A Comprehensive Inspection
Brake assembly diagramA complete brake job should restore the vehicle's brake system and braking performance to good-as-new condition. Anything less would be an incomplete brake job. We perform the following:

  1. A complete brake job begins with a thorough inspection of the entire brake system; lining condition, rotors and drums, calipers and wheel cylinders, brake hardware, hoses, lines, and master cylinder.

  2. Any hoses that are found to be age cracked, chaffed, swollen, or leaking are replaced. We use the same type of end fittings (double-flared or ISO) as the original, so fitting types are not intermixed.

  3. Steel lines that are leaking, kinked, badly corroded, or damaged must also be replaced. For steel brake lines, we use only approved steel tubing with double-flared or ISO flare ends.

  4. A leaking caliper or wheel cylinder needs to be rebuilt or replaced. The same applies to a caliper that is frozen, damaged or badly corroded.

  5. Leaks at the master cylinder or a brake pedal that gradually sinks to the floor you indicates that the master cylinder needs replacing.

  6. The rotors and drums are inspected for wear, heat cracks, warpage, or other damage. Unless they are in perfect condition, they will be resurfaced before new linings are installed. If worn too thin, we replace them.

  7. Rust, heat, and age have a detrimental effect on many hardware components. It's a good idea to replace some of these parts when the brakes are relined. On disc brakes, new mounting pins and bushings are recommended for floating-style calipers. High temperature synthetic or silicone brake grease (never ordinary chassis grease) should be used to lubricate caliper pins and caliper contact points.

  8. On drum brakes, shoe retaining clips and return springs are replaced. Self-adjusters are replaced if they are corroded or frozen. We use brake grease to lubricate self-adjusters and raised points on brake backing plates where shoes make contact.

  9. Wheel bearings should be part of a complete brake job on most rear-wheel drive vehicles and some front-wheel drive cars. Unless bearings are sealed, they need to be cleaned, inspected, repacked with wheel bearing grease (new grease seals are a must), and properly adjusted.

  10. As a rule, tapered roller bearings are not preloaded. Finger tight is usually recommended. Ball wheel bearings usually require preloading.

  11. As a final step, old brake fluid should always be replaced with fresh fluid.

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About Brake Lining Replacement

Don't Mix or Substitute Brake Lining Types
Replace semi-metallic brake linings with semi-metallics, nonasbestos organic (NAO) linings with nonasbestos, and asbestos linings with either asbestos or NAO.

Asbestos linings should never be substituted for semi-metallics. Asbestos in such an application will wear too quickly and may not provide the stopping power necessary for safe braking.

Front-wheel Brakes Might Have Different Requirements

Semi-metallic linings are usually required on the front brakes of front-wheel drive cars because of higher operating temperatures. Front brakes typically handle up to 85% of the brake load. Semimetallic linings help conduct heat away from brake rotors. Asbestos does not because it is an insulator.

If asbestos linings are used in a FWD application, brakes may run too hot. Since heat is directly proportional to lining life, asbestos pads won't last.

Quality Versus Cost

Most friction suppliers offer a choice between economy and premium replacement linings. Some also offer a midrange lining. Most people either want the best replacement linings or the cheapest. We recommend a quality brand of premium lining because it will last longer and generally provide better braking performance.

To assure maximum performance, quiet operation and long life, rotors and/or drums should be resurfaced when linings are replaced. We also recommend shims or an antisqueal compound for application to the backs of disc brake pads. These help dampen vibrations to quiet brake squeal.

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Brake System Symptoms Checklist

If you are having brake system problems, it will help us to know the symptoms. We have provided a checklist which contains the questions that will help our technicians help you. It would be the easiest, if you would fill out the form and press the Send button. If you prefer, you can print out the form and fill it out by hand or fill it out on-line and print it. Then bring it in with you when you come into our shop.

Display Brake Symptom Checklistchecklist.

Brake System Diagram

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Brake Cleaning Solvents

Brake cleaner is a chemical product specifically formulated to do a specific job. There are other chemicals that can be used for cleaning, but many leave behind a residue that can contaminate linings and attack rubber components. Brake cleaners contain a variety of ingredients. Many have been reformulated to make them more environmentally friendly.

For years, 1-1-1 trichloroethane was the primary ingredient in most aerosol brake cleaners. It works great, dries almost instantly, leaves no residue and is nonflammable.

It was also a relatively inexpensive chemical until a new federal excise tax increased its cost. The tax was applied because 1-1-1 trichloroethane is an ozone-depleting chemical (like CFCs).

To minimize the impact of the new tax, some cleaners now use a mixture of 1-1-1 trichloroethane with other chemicals such as perchloroethylene. Others have eliminated 1-1-1 trichloroethane altogether and use other chemicals.

Perchloroethylene is a chemical used by dry cleaners because of its excellent cleaning properties. It dries fast (though not quite as fast as 1- 1-1 trichloroethane), leaves no residue, and is nonflammable. It also costs less than 1-1-1 trichloroethane.

Perchloroethylene is a Volatile Organic Compound (VOC), which is being regulated because it contributes to air pollution. It is also toxic and not biodegradable. Some cleaners use nonchlorinated formulas, including n-methyl pyrollidone, oxodecyl acetate (acetone), methanol alcohol, toluene, and various petroleum distillates. None of these chemicals are CFCs and are exempt from federal CFC taxes.

Most nonchlorinated brake cleaners don't clean as well as 1-1-1 trichloroethane or perchloroethylene, nor do they dry as fast. Products without petroleum distillates do not leave a residue.

Nonchlorinated brake cleaners are flammable, making them subject to a national fire code regulation limiting the number of cans of flammable product a retail store or parts jobber can stock. Nonchlorinated brake cleaners are also VOCs and not biodegradable. Some are toxic.

Another alternative are water-based brake cleaners. These typically contain a citrus-based solvent, such as D-Limonene, and other ingredients, such as Methyl Ethyl Ketone. The primary advantages with a water-based formula are nonflammability, no residue, biodegradability of the base product, low VOCs, no ozone depletion or greenhouse gases, and low toxicity. Water-based products clean more slowly and may require scrubbing or wiping to do a thorough job. Drying times are also slow, measured in hours rather than minutes or seconds.

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Rotor Drum Turning Issues

Rotor drum turning diagramIf a customer wants drums turned to a size outside the limits cast into the drum, you must refuse. They cannot be turned thinner than the minimum thickness specifications stamped or cast on the rotor or drum itself. A drum or rotor worn or turned too thin may not be able to absorb and dissipate heat quickly.

This can make the brakes run hot, accelerate lining wear, and reduce braking effectiveness. It can also lead to rotor or drum warpage and a pulsating brake pedal. Most drums are cast with enough thickness to allow 0.090" of wear. In other words, the difference between a drum's diameter when new and its discard diameter is 0.090," but that doesn't mean you can machine a drum right up to the 0.090" limit. You should never turn a drum that's worn more than 0.060" beyond its original diameter.

The 0.060" limit leaves a 0.030" margin for additional wear. If you turn a drum that's worn more than 0.060," or if the drum ends up being more than 0.060" larger after turning, there may not be enough metal left to handle normal wear until the next brake job.

The 0.090" discard limit is the maximum acceptable wear the drum can safely handle before the metal is too thin. Any drum worn beyond 0.060", or that would be over 0.060" larger after resurfacing, should never be turned on a lathe, it should be replaced.

Wear is checked by measuring diameter with a drum micrometer. If the gauge shows enough metal left to safely turn it, the drum can be resurfaced to restore and true the surface.

Like drums, the amount of wear a rotor has experienced will determine whether or not it can be resurfaced. The two-key rotor dimensions to take into account are minimum refinish thickness and discard thickness.

Discard thickness is usually cast in the rotor itself, but minimum refinish thickness must often be looked up in a reference manual or brake specification chart. Minimum refinish thickness is the limit for resurfacing the rotor. If the rotor has worn to the point where its thickness will be less than the specified dimension after resurfacing, the rotor should be replaced.

Discard thickness is the maximum acceptable wear limit. Once the rotor is worn beyond discard thickness, it must be replaced. The difference between discard and minimum refinish thickness is the margin the vehicle manufacturer believes is necessary to allow for normal wear between brake jobs. It varies considerably from one vehicle manufacturer to the next, and according to vehicle size and type of brakes used.

The margin specified on most domestic passenger cars is around 0.015." The range is 0.020" to 0.030" for most imports. A few, such as Jaguar, have as much as a 0.050" difference between minimum refinish thickness and discard thickness.

Thickness should be measured with a micrometer at six evenly spaced points around the rotor. The smallest measurement should be used since this is how far the rotor will have to be machined to restore the surface.

Measuring at various points around the rotor will reveal any variations in rotor thickness or parallelism. Both surfaces of the rotor must be within the manufacturer's specified tolerances for parallelism, otherwise the rotor can cause excessive pedal travel (by kicking the pads too far out as it turns), front end vibration, pedal pulsation, and chatter.

Parallelism specs recommended by various vehicle manufacturers range from as low as 0.0001" to as high as 0.0008." Refer to reference charts to determine how much correction, if any, is needed.

Another critical rotor dimension is runout. Lateral runout is the movement of the rotor from side to side as it turns. Excessive runout will kick the pads out as the rotor turns, creating excessive clearance requiring increased pedal travel when brakes are applied.

Runout specifications vary from as low as 0.002" to as high as 0.006." You should always refer to the particular specs listed by the vehicle manufacturer when checking runout.

Runout is checked with a dial indicator while the rotor is still on the car. If run-out exceeds the recommended limit, the rotor must be resurfaced or replaced. Drums and rotors should always be inspected for heat cracks, distortion, damage, and hard spots prior to resurfacing. Cracks, damage and hard spots call for replacement. If distortion can't be eliminated within the limits of resurfacing, replacement will also be necessary.

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The Difference Between Four Wheel ABS and Rear Wheel ABS

The Difference
ABS Comparison DiagramFour wheel ABS is used on passenger cars and some light trucks and vans. Rear wheel ABS is used only on trucks. Rear wheel anti-lock systems are typically used on applications where rear wheel traction is affected by vehicle loading. Rear wheel ABS systems are simpler and less costly than their four wheel counterparts.

On a four wheel application, the ABS system keeps track of wheel deceleration rates with wheel speed sensors. Some have one speed sensor at each wheel while others use a common sensor in the differential or transmission for both rear wheels.

With rear wheel ABS, only a single wheel speed sensor in the differential or transmission is used for both rear wheels.

Four wheel ABS systems include those made by Bendix, Bosch, Delco Moraine, and Teves. Most rear wheel ABS systems are made by Kelsey-Hayes, though Kelsey-Hayes also makes some four wheel systems.

Kelsey-Hayes rear wheel ABS systems have been in use since 1987 on Ford F series trucks, as well as later model Ranger, Bronco, Bronco II and Explorer trucks and Aerostar vans. Ford calls their version the Rear-wheel Antilock Brake System or RABS system.

On General Motors applications, it is called the Rear Wheel AntiLock or RWAL system. It is on '88 and later Chevrolet "C" and "K" series pickups, '89 "M" series (Astro) minivans and "S" and "T" series pickups, some "S" series Blazers, and '90 to '92 "R" and "V" series light trucks and "G" series vans. Dodge has used the RWAL system since 1989 on its "D" and "W" 150/350, Dakota and Ram Charger pickups.

Kelsey-Hayes RABS and RWAL systems are nonintegral rear wheel only antilock brake systems. The conventional master brake cylinder and power booster supply brake pressure to a dual solenoid control valve for the rear brakes.

The ABS control module receives a speed signal from a single vehicle speed sensor. On Ford and Dodge applications, the sensor is in the differential. On GM, it is located in the transmission tailshaft.

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Why It Is Important to Periodically Replace Brake Fluid

Brake Fluid Absorbs Water
Most brake fluids are glycol based and absorb moisture over time. Moisture contamination causes the fluid boiling point to drop, which could lead to fluid boil and brake fade in some circumstances.

This is so important to safety that the Department of Transportation requires that brake fluids meet certain specifications. The two main Department of Transportation specifications are DOT 3 and DOT 4. Among other parameters, the DOT 3 specification requires the minimum "dry" (contains no water) boiling temperature to be 401° F and the minimum "wet" (saturated with water) boiling temperature to be 284° F.

The DOT 4 specification requires the minimum dry boiling temperature to be 446° F and the wet boiling temperature to be 311° F.

Moisture contamination also contributes to internal corrosion in the calipers, wheel cylinders, and steel brake lines. The reason for replacing fluid periodically is to get rid of contaminated fluid and restore fluid heat and corrosion resistance.

Moisture seeps in through microscopic pores in rubber seals and hoses. It also enters every time someone opens the master cylinder reservoir to check the fluid level. Most fluid reservoirs are transparent so level can be checked without having to open the lid.

Brake fluid is so hygroscopic (attracts water) that leaving the lid off a can of fresh brake fluid can ruin it overnight. It will absorb so much moisture from the air that it becomes too badly contaminated to use.

How often should the fluid be replaced? By the time a new car is only 12 months old, its brake fluid contains about 2% water. After 18 months, the water content is approaching 3%, which is enough to lower the boiling temperature by 25%. After several years of service, it is not unusual to find brake fluid containing seven to 8% water. For this reason, many experts recommend replacing the fluid as a preventative maintenance service every two years or 24,000 miles. At the very least, it should be replaced when brakes are relined.

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Rotor Surface Finishes

Machined surface finishes are usually measured with "Surface Profilometers." But for any given Rotor surface you can use a Relative Machined Surface (RMS) measurement. For example:

With the introduction of Anti-Lock Braking Systems, brake rotor finish tolerances are more critical. Manufacturer's are now providing surface finish specifications. They will specify, for example, a brake rotor surface finish of 60 RA or Less" where RA is "Roughness Average."

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Turning New Rotors or Drums

Two Points of View
There are two conflicting points of view on this issue. Some manufacturers say it is not necessary because their drums and rotors are finished at the factory and come ready to install. Resurfacing is therefore totally unnecessary. Furthermore, it would reduce the thickness of the drum and rotor and consequently its overall service life.

Others say that in spite of what the drum and rotor manufacturers say, new drums and rotors should always be turned to make sure they are true and flat.

It is better for you and your customer to take a thin cut to make sure everything is right rather than risk a comeback because of noise or pedal pulsation.

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