|The Brake System - Maintenance|
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.
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.
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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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.
There are several common methods in use:
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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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.
Another 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.
A 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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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.
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.
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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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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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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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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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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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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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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