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Basic Brake System Service

Any time your vehicle needs brake work, you should ask yourself, "Is this something I really want to do myself?" If you don't feel competent enough to tackle a do-it-yourself brake repair, don't! Brakes are a critical safety item that must be serviced correctly. Don't take chances if you have doubts about your own capabilities. Pay a professional to fix your brakes. But hey, this is a brake book for do-it-yourselfers, so you probably won't even be reading this section unless you were ready to do the work yourself. So let's proceed and get through the preliminaries before we tell you how to do first class brake repairs.

Brake work is not difficult to do, but there are some safety precautions you must keep in mind before you proceed. Brake workbrake caliper requires you to raise your vehicle so the wheels can be removed to provide access to the brakes. This sounds simple enough but can create a potentially dangerous situation if not done correctly:
* First, work on a level surface to minimize the risk of your vehicle rolling or moving when the wheels are raised off the ground.
* Second, place the transmission in Park (automatic) or in gear (manual) to prevent your vehicle from rolling before you jack it up. If you're not going to work on the rear brakes or raise the rear wheels off the ground, set the parking brake, too. But if you need to work on the rear brakes, use chocks to block the wheels.
* Loosen the lug nuts before you raise the wheels off the ground. This makes the job much easier and eliminates the danger of pushing or rocking the vehicle off the jack.
* Place a pair of sturdy safety stands under the chassis as soon as the vehicle is raised. The stands should be positioned under a structural component such as the frame rails, engine cradle or some other solid point that isn't going to move. Do this before you remove the wheels and before you start to work on the brakes. Never, repeat never, crawl under a vehicle that is supported only by a jack. Relying on a bumper jack or floor jack alone to hold a vehicle up is gambling with your life!
* Avoid exposure to brake dust. Your brakes will be dirty and coated with dust from the worn linings. The dust may contain asbestos, and even if it doesn't brake dust isn't something you want to breathe. When the wheels are removed, wash off or clean the brakes with brake cleaner. Some aerosol brake cleaners contain toxic or carcinogenic chemicals, so note anybrake caliper precautions on the product's label before using (like warnings to use only in a well ventilated area). Also, never blow off the brakes with an air hose because this will force the dust into the air.
* Heed all the warnings and cautions throughout this book. Where various procedures or repairs might get you into trouble, we've tried to alert you to the potential dangers.
* If your vehicle has Hydro-Boost or integral ABS with electrohydraulic power assist (Teves Mark 2 ABS, Bosch III ABS, Delco Powermaster 3 ABS, Bendix 10 and Jeep ABS), you must fully depressurize the high pressure accumulator before starting any brake work. On Hydro-Boost applications, this is done by depressing the brake pedal 4 to 6 times with the engine off. On the integral ABS applications, you have to depress the brake pedal 30 to 40 times with the ignition off.
* Be careful not to splash brake fluid on your vehicle's finish. Brake fluid can cause paint to streak.
* Wear eye protection if you do any pounding or chiseling.

DOING A COMPLETE BRAKE JOB Now that we've covered the safety precautions, let's move on to what's involved in doing brake work. More specifically, let's talk about the importance of doing a "complete" brake job.

A complete brake job is the opposite of an "incomplete" brake job that overlooks things or leaves some things undone. We're talking about ignoring a leaky caliper or wheel cylinder that needs to be rebuilt or replaced, reusing old drum hardware that's badly rusted or weakened with age, forgetting to lubricate shoe pads and caliper mounts so these parts can move freely, reusing old grease seals to save a few bucks, not changing the brake fluid or bleeding the lines because it's takes too much effort, not even checking the condition of the rear brakes because you're only replacing the pads up front. Things like these can get you intro trouble -- if not now, then at some point down the road.

So anytime your vehicle needs brake work, think in terms of doing a complete brake job. That doesn't mean you have to overhaul the entire brake system if it only needs a new set of linings. But it does mean doing a thorough inspection of your entire brake system so you can identify any additional items that might need adjustment, repair or replacement, and also doing those things that are necessary for preventative maintenance (like lubricating the caliper slides, shoe pads, changing the fluid and bleeding the lines). This may sound like a lot of unnecessary extra work, but it will usually save you money and trouble in the long run because it helps to prolong the life of your brake system while assuring optimum brake performance and safety. Let's say your vehicle needs new pads up front. There's no need to replace the rear shoes as long as the rear linings are in good condition. But if the shoes are marginal or have been contaminated by fluid leaks or grease, they also need to be replaced -- along with any other drum components that are not in good condition. Even if the shoes are still within service limits, you should ask yourself if they'll last as long as the pads you're replacing up front. If not, then now might be a good time to replace them.

Whether or not the calipers and wheel cylinders need attention will depend on their condition. Any sign of leakage or sticking calls for immediate repair or replacement. And even if they're not leaking, sticking or frozen, you should consider rebuilding or replacing them for preventative maintenance.

One of the pitfalls of do-it-yourself brake work is that you don't have anyone looking over your shoulder to hold you accountable for what you do or don't do. A professional brake technician has his employer and customer to worry about as well as his own reputation. In some states, there may also be a regulatory agency that serves as an auto repair watchdog. In any event, a professional has to guarantee his work. If a problem occurs during the warranty period, he has to fix it for free. So there are numerous incentives to do the job right the first time.

As a do-it-yourselfer, on the other hand, you are your own boss. Except maybe for your spouse or a nagging mother-in-law, you're accountable only to yourself and your own conscience. Even so, it's important to remember what's at stake. You don't want to take chances with your brakes because the consequences of doing a halfhearted job could be deadly. So the responsibility is yours for deciding what needs to be fixed and what ought to be fixed for preventative maintenance. Repairs that should always be made include any brake parts that are worn out, broken or damaged. Brake linings worn down to minimum specifications, for example, would need to be replaced because they might not be able to create enough friction to safety stop your vehicle. They may also damage your rotors or drums, which will end up costing you even more money. It's the same story if you find a leaky caliper, wheel cylinder, master cylinder, brake valve, hose or steel line. Leaky components must be repaired or replaced because fluid loss could lead to brake failure.

Every brake job should start with a thorough inspection of the entire brake system, including the antilock brake system if your vehicle is so equipped.

* First, start with a warning light check. Turn on the ignition to verify that the brake warning light (and ABS warning light if your vehicle has ABS) comes on. The light(s) should come on momentarily for a bulb check, then go out if no problems are present. No light? Then you've found a bulb that needs replacing or a wiring problem. If the light comes on and remains on (does not go out), then further diagnosis will be required to find out what's wrong.
* Apply the brakes and start the engine. Does the pedal drop slightly? That's good because it indicates a good vacuum booster. No boost may indicate a leaky booster diaphragm or vacuum connection.
* How does the brake pedal feel? Is it firm? A soft or mushy-feeling pedal usually indicates air in the lines or leaks. A pedal that slowly sinks is a classic symptom of a worn master cylinder. Is the amount of pedal travel normal? A low pedal may indicate worn linings, the need for adjustment, defective or frozen drum brake adjusters or a low fluid level.
* Check your stop lights (taillights & center high mounted stop light. Do all the lights come on when you press on the brake pedal? No lights may indicate a defective or misadjusted stop light pedal switch, a wiring problem or burned out bulbs in the taillights.
* Apply the parking brake. Does the pedal or handle work smoothly? Is it adjusted properly? Does the brake light come on? No brake warning light may indicate a bad bulb or defective or misadjusted parking brake switch. Does the parking brake hold the vehicle? Put the transmission into gear while the parking brake is applied. If should hold the vehicle with the engine idling. If it fails to hold the vehicle, it may need adjustment or repair. Now release the parking brake. Failure to release fully means the linkage or cables need attention.
* Turn the engine off, place the transmission in park (or gear if you have a manual), open the hood and check the fluid level in the master cylinder. A low level may indicate a leak or worn linings. Also, note the fluid's appearance. Dark discoloration indicates moisture contamination and the need for a fluid change.
* If you're working on somebody else's vehicle or one that you haven't driven for some time, take it for a short drive to test the brakes.

WARNING: Do not test drive any vehicle that does not have a full pedal, adequate brake fluid or enough brakes left to safely bring it to a halt!

Try a couple of low speed gradual stops. How do the brakes feel? Do you feel any pulsations in the pedal? Pulsations usually indicate warped rotors that need to be resurfaced or replaced. Does the pedal feel soft or spongy? There could be air in the lines? Is pedal travel excessive? The brakes could be worn or need adjusting. Do the brakes pull to either side? Uneven braking could indicate fluid or grease contaminated linings, or a bad caliper or wheel cylinder. Do you hear any unusual noises? Scraping sounds could indicate badly worn linings or loose debris inside a brake drum. Squealing may indicate worn or loose pads. Do any warning lights come on when you apply the brakes? A warning light could signal loss of pressure in one of the hydraulic circuits.

If the brakes are working normally, try a panic stop from about 35 to 40 mph. WARNING: Do not attempt this if you're already detected a brake problem or the brakes are defective. Again, note how the brakes behave. Braking should be even, the pedal should remain firm and there should be no unusual noises or sensations. If the vehicle is equipped with ABS, you may feel pulsations in the pedal and hear clicking or buzzing noises from under the hood. This is normal for most ABS systems and does not indicate a problem. The ABS system should prevent the wheels from locking up and allow you to make a safe, controlled stop. If the vehicle does not have ABS, the rear wheels may lock up and skid depending on how hard you brake and traction conditions. This too is normal for vehicles that do not have ABS.

* Remove a front wheel and measure the thickness of the brake pads. If worn down to minimum specifications or if wear indicators are making contact with the rotor, new linings are needed. If the pads are still above specs, you might consider replacing them anyway if they're near the end of their service life or if they're noisy.
* Note the condition of the rotors. Deep scratches or grooves indicate a need for resurfacing. Measure runout and parallelism, too. If out of specs, resurfacing or replacement is needed. Are there discolored spots, heat cracks or warpage? These may also indicate a need for rotor resurfacing or replacement.
* Note the condition of the calipers and caliper mounts. If the pads are worn unevenly, the caliper is hanging up and is not centering itself over the rotor. This would indicate a need for disassembly, cleaning (or replacing) and lubricating the caliper slides or bushings with brake grease. If you see any signs of fluid leakage around the caliper piston seal, it would tell you the caliper needs to be rebuilt or replaced.
* Check the condition of the rubber brake hose. If cracked, frayed, worn, damaged or leaking, it needs to be replaced.
* Pull a drum and inspect the drum surface, brake shoes, hardware and wheel cylinder. If the shoe linings are at or below minimum specifications, you need new shoes. If the linings are still above minimum specs but are getting thin, consider replacing the shoes anyway. Any wetness or leakage around the wheel cylinder means it needs to be rebuilt or replaced.
* Check everywhere for leaks: the master cylinder, proportioning valve, steel brake lines, all rubber hoses, and ABS components if your vehicle has ABS. Have a helper apply the brakes while you look underneath. Rubber hoses should not expand under pressure. Any hoses that are swell under pressure need to be replaced.

* Always reline brakes in matched axle sets. In other words, do both front wheels, both rear wheels or all four-wheels. Never do just one wheel on the right or left side as this can cause the vehicle to pull towards one side when braking. Also, use the same type, grade and brand of friction material on both sides. Mismatching friction material side-to-side may also cause uneven braking and a brake pull. Keep this in mind if you're buying a single "loaded" caliper assembly that comes complete with new pads. You should also replace the pads on the other caliper so both will have the same friction material.
* If you're replacing brake linings, use the same type of material as the original. Replace semi-metallics with semi-metallics, never asbestos or NAO. Replace NAO with NAO. Replace asbestos with asbestos or NAO. You can also upgrade from from asbestos or NAO to semi-metallics if the friction supplier offers such linings for your vehicle application.
* Premium quality linings will give you the best wear life, noise control and brake performance. If you can't afford the best, then go with standard grade replacement linings. Avoid economy grade linings.
* Have your rotors and/or drums resurfaced when relining the brakes unless they're in perfect condition. New linings require a smooth surface to rub against for maximum braking effectiveness. A rough or grooved rotor or drum will wear linings rapidly and reduce braking effectiveness. Warped rotors or ones with excessive runout can also cause annoying pedal pulsations.
* Rotors and drums should always be resurfaced in pairs. As a rule, there should be no more than .010 in. difference in rotor thickness or drum diameter side-to-side.
* Rotors or drums that are worn down to minimum specifications, or cannot be resurfaced without exceeding the minimum or discard specifications must be replaced.
* New hardware is recommended for drums and calipers. Replace hardware items such as the retaining clips and return springs. Heat and age weakens these components. Self-adjusters should also be replaced if found to be corroded or frozen. Use high temperature brake grease (never ordinary chassis grease!) to lubricate the self adjusters and the raised shoe pads on the backing plates. New mounting pins and bushings will help keep disc brake calipers sliding freely. Slides and bushings should also be lubricated with high temperature brake grease.
* Don't overlook the wheel bearings when relining the brakes. Check for looseness by rocking the wheel in and out. Check for roughness or noise by spinning the wheel by hand. Greasable wheel bearings should be cleaned and inspected. If worn, cracked or damaged, the bearings need to be replaced. If good, the bearings need to be repacked with high temperature wheel bearing grease. Never reuse old grease seals (they usually leak). When wheel bearings are reinstalled, they must be set with the correct amount of preload (not too tight or too loose). Wheel bearing adjustment procedures vary, so always refer to your shop manual for the specifics on your vehicle. Note: Though most front-wheel drive vehicles have sealed front wheel bearings, the rear wheel bearings on many are greasable -- and often neglected.
* It is not absolutely necessary to rebuild or replace disc brake calipers or drum brake wheel cylinders when doing a brake job -- as long as these components are not leaking or sticking. Even so, most experts recommend rebuilding or replacing them anyway for preventative maintenance.
* Always replace the old brake fluid with fresh fluid after relining the brakes, and bleed the brakes to remove air bubbles from the lines. Brake fluid absorbs moisture over time, which lowers its boiling temperature and promotes internal corrosion. Replacing the fluid every couple of years for preventative maintenance or when relining the brakes will prolong the life of the hydraulic components while assuring maximum brake performance and safety. Never reuse old brake fluid or fluid from a container that's been left open for several days or longer. The fluid will be contaminated by airborne moisture and unsuitable for further use.
* Always use a torque wrench to final tighten wheel lug nuts. This is a much more accurate method than using a four-way wrench, breaker bar, ratchet or impact wrench.
* Break in new linings gradually. Avoid sudden stops or hard braking for the first 150 to 200 miles. This will minimize the risk of glazing new pads.

How often should you check your linings? It depends on the type of driving you do. Checking the linings every six months or once a year would be a general guideline. Stop and go city driving will wear the linings much more quickly than open highway driving because the brakes are used more often. So if most of your driving is down in town, you might want to check the linings more often. The same holds true if you do a lot of driving on hilly or mountainous terrain. And if you have an aggressive driving style or do any weekend racing, better plan on checking your brake linings on a much more frequent basis.

Disc brake pads can be easily inspected by removing a front wheel, but the rear drums also require pulling a drum. It's difficult to accurately judge lining wear by appearance alone, so always measure the exact thickness of the linings at their narrowest point to determine whether or not they're still within acceptable limits. If the linings are at or less than the minimum thickness specification (refer to a shop manual), you need new linings. Don't put off replacing the linings too long because the minimum thickness is based on the design of the brake system (how far the caliper pistons, wheel cylinder pistons and self-adjusters can safely travel) and the type of linings used (bonded, molded or riveted). Bonded and molded linings can generally tolerate more wear than riveted linings. But if wear either type too far, you can chew up your rotors or drums. New linings would also be needed if the wear indicators on your disc brake pads are making contact with the rotors. The indicators will make a loud high pitched squeal to warn you it's time to replace the pads.

The linings should also be replaced if they are found to be contaminated with brake fluid or grease (regardless of wear), or if they show uneven or taper wear. Replacement would be needed if the difference in thickness from one end of the pad to the other is more than 1/16th inch (1.5 mm) on fixed calipers, or 1/8th inch (3.0 mm) on floating calipers.

Resurfacing rotors and drums unnecessarily will obviously reduce their useful service life, but in many instances resurfacing is necessary by the time the linings are replaced. The general rule here is to resurface when necessary, and don't resurface when it isn't necessary.

As long as your rotors and drums are in relatively good condition (smooth and flat with no deep scoring, cracks, distortion or other damage), there's no need to resurface them when doing a brake job. But they're not in good condition or you've had a problem with noise, resurfacing is recommended.

The factors that determine whether or not resurfacing is really necessary include:

* Surface condition. This is the most important criteria. Scoring, pitting or other minor surface imperfections should be cleaned up to restore the surface to like-new condition and to minimize noise. If a rotor or drum is cracked, however, it must be replaced.
* Lateral runout on rotors. This must be measured with a dial indicator against the face of the rotor. If rotor wobble exceeds OEM specs, re indexing it on the hub (if possible) may help reduce runout. A better solution here is to have the rotor resurfaced Brake Service in place with an "on-car" lathe. Many vehicle manufacturers now recommend this technique, which unfortunately requires expensive equipment that only a dealer or brake shop would have. But the equipment does a fantastic job and cuts the rotor true to its axis of rotation.
* Rotor warpage. Variation in the thickness of the rotor or uneven spots on either rotor face will cause the brake pedal to pulsate or shudder when the brakes are applied. Flatness can be checked by placing a straight edge against both faces of the rotor. Thickness must be checked with a micrometer at six or more points around the rotor. If parallelism between rotor faces exceeds OEM specs (generally about .0005 in.), or if the rotor is warped or has hard spots (which are often discolored blue or black), the rotor should be resurfaced or replaced. Hard spots that develop from overheating or uneven tightening of lug nuts can create raised areas on the surface that often extend below the surface. The metallurgical changes in the rotor often cause the hard spots to return after a few thousand miles so replacing the rotor may be the best long term fix.
* Rotor thickness and drum diameter -- If a rotor is close to or at the minimum thickness specification (which is stamped on the rotor or may be found in a brake service reference book), it is too thin to be resurfaced and must be replaced. The same goes for drums, except the critical dimension here is the drum's inside diameter (the maximum or discard diameter spec is stamped on the drum or listed in a reference book), Most experts today recommend a rotor finish of 40 to 60 micro inches or smoother for quite disc brake operation. This is achievable with most bench lathes or on-car refinishing equipment. When you have your rotors or drums resurfaced, ask them to remove the least amount of metal possible to maximize the remaining service life of your parts. Sanding rotors after they've been turned with #120 to #150 grit sandpaper will leave a smooth, non directional finish that provides an optimum surface for semi-metallic pads. Rotors should also be scrubbed with a brush and soapy water before they are reinstalled. Resurfacing leaves a lot of metallic debris on the surface which can embed itself in the new brake linings and cause noise problems. Even if the rotors or drums have not been resurfaced, cleaning is recommended to remove dirt and grease (which can contaminate new linings and cause uneven braking or grabbing).

As we've said already, it isn't absolutely necessary to rebuild or replace your calipers when relining the brakes. But not doing so may be asking for trouble later on. A caliper that's leaking brake fluid, is damaged, has a frozen piston, or is causing uneven pad wear obviously needs to be rebuilt or replaced. But what if your calipers aren't leaking? Should you rebuild them anyway for preventative maintenance?

Those who subscribe to the "don't fix it unless it's broken" philosophy of brake repair think they're saving time and money by leaving the calipers alone. Maybe so. But most brake experts say it makes sense to go ahead and do the calipers when the brakes are relined. Here's why:
Brake calipers, like any other mechanical component, wear and corrode with age. Every time the brakes are applied, the back-and-forth motion of the caliper pistons produces a slight amount of wear. At the same time, moisture is building up in the fluid. After several years of service, the fluid may contain as much as 3% water -- which is enough to produce visible corrosion in the caliper bores and on steel pistons. As the surface of the pistons become rough, they scour the seals with every application of the brakes. Eventually this will lead to fluid leaks and pad contamination.

So even if a caliper isn't leaking, it's still aging inside. Rubber piston seals and dust boots harden and become brittle over time. One of the jobs the piston seals do besides keep the fluid where it belongs is to help retract the pistons when the brakes are released. When a piston moves out, it twists the square cut seal slightly. This helps pull the piston back when the brakes are released to keep the pads from dragging against the rotors. But as a seal ages and loses elasticity, it doesn't do as good a job of pulling back the piston. Consequently, the pads start to drag resulting in reduced fuel economy, a possible brake pull and/or accelerated or uneven pad wear.

When boots get old, they often crack or split, allowing dirt and water to enter the piston bore area. The result can be accelerated seal wear, piston corrosion and sticking. What's more, if the pistons are pushed back in to accept new pads any dirt that's found its way behind the boots will be shoved back into the caliper bores. This too, can contribute to sticking, binding and wear.

So there are valid reasons for rebuilding or replacing the calipers when the brakes are relined.

The same arguments that apply to disc brake calipers also apply to the wheel cylinders in drum brakes. As long as they're not leaking, sticking or damaged, you might be tempted to leave them alone and take your chances. But the seals inside are aging, so rebuilding them for preventative maintenance will assure trouble-free operation.

A complete brake job also includes fresh brake fluid. Bleeding is necessary for two reasons:

1. To remove air bubbles that may have entered the system while repairs were being made, because of a leak or because the fluid level got too low. The air must be removed because it is compressible and can prevent a full, firm pedal.
2. To remove moisture contamination. Brake fluid absorbs moisture over time, which lowers its boiling point and contributes to internal corrosion. Changing the fluid periodically (every two years or when the brakes are relined) as preventative maintenance rids your brake system of unwanted moisture, restores the fluid's boiling temperature and prolongs the life of the hydraulic components by minimizing the potential for internal corrosion. This can be a really money saver if your car or truck has ABS because of the high replacement cost of the hydraulic modulator assembly. As a general rule, the sequence or order in which the individual brakes are bled is very important to get all the air out of the system. The sequence varies from one vehicle to another, so always refer to the bleeding sequence specified by your vehicle manufacturer. You'll find this information in a shop manual.

The most common procedure on hydraulic systems that are split front-to-rear (most rear-wheel drive cars and trucks) is to do the rear brakes first, then the fronts, starting with the wheel furthest from the master cylinder. With systems that are split diagonally (most front-wheel drive cars and minivans), the sequence is often right rear, left front, then left rear and right front. Always use the type of brake fluid specified by the vehicle manufacturer (usually DOT 3, but DOT 4 in most European cars). DOT 5 silicone fluid is not recommended for vehicles equipped with ABS. Use clean fluid from a sealed container, and discard the old fluid.

* Always use the type of brake fluid that's specified by the vehicle manufacturer (usually DOT 3 but sometimes DOT 4).
* Avoid contact with DOT 3 and 4 brake fluids. It can cause eye and skin irritation, and may be harmful or fatal if swallowed.
* Be careful not to splash or spill DOT 3 or 4 brake fluid on painted finishes. It acts like a solvent and attacks the paint. If you do spill, don't wipe. Flush immediately with water.
* Use only fresh brake fluid from a sealed container. Never reuse old fluid or brake fluid from an open or unmarked container.
* Don't leave brake fluid containers open. Close the lid immediately after use to keep out moisture. The same goes for the master cylinder fluid reservoir. Don't leave it open unless you are adding fluid or bleeding the brakes.

Anytime the brake system is opened for repairs or to replace a hydraulic component such as a caliper, hoses, wheel cylinder, etc., the system should be bled to purge all the air from the lines. Air is compressible so you don't want any in the fluid. So you may have to bleed the entire brake system even if you only opened or replaced a single line. Air bubbles can quickly migrate through the system, so in most cases you can plan on bleeding all four brakes.

A timesaving trick that can eliminate the need to bleed all four brakes is to clamp off the rubber hose to the brake prior to opening the line to replace a caliper or wheel cylinder. The clamp will prevent air from backing up through the open line. When you're done, just remove the clamp and bleed the one line.

When bleeding the brakes (or changing the brake fluid), any of the following techniques can be used: manual bleeding, gravity bleeding, pressure bleeding or vacuum bleeding.

Regardless of which method you use, the hardest part of bleeding the brakes is loosening the bleeder screws without breaking them off. If it's been years since the brakes were last bled, chances are the bleeder screws will be difficult to loosen. Apply penetrating oil and allow it to soak overnight. The careful application of heat with a propane torch may help loosen stubborn screws. Using a six-point bleeder socket or wrench is also a good idea to prevent rounding off the tiny screw. If a screw twists off, you have two options: forget it and move on to the next one, or replace it. If you can bleed the other lines and get a firm pedal, it may not be necessary to replace the broken bleeder screw. But if there's air in the line and the pedal remains soft or won't come up, then you'll have to replace the broken screw. This usually requires removing and replacing the caliper or wheel cylinder unless you can get the base of the broken screw out with a reverse-twist extractor bit. Drilling out the broken screw and re tapping the hole is also possible but also requires removal and/or disassembly of the caliper or wheel cylinder so you don't end up with bits of metal inside the hydraulic system.

* Manual bleeding -- With this procedure you use the brake pedal to push fluid through the lines. The "bleeder screws" on the calipers and wheel cylinders are opened one at a time (in the specified sequence, usually starting with the brake furthest away from the master cylinder). A "bleeder hose" is then attached to the bleeder screw and immersed in a container partially filled with brake fluid. The pedal is then slowly depressed to push fluid through the line to remove air. When the fluid emerging from the bleeder screw is clear (no bubbles or discoloration), the bleeder screw is closed to prevent air from being drawn back into the line. Check the fluid level in the master cylinder after bleeding each brake to make sure it doesn't run low and allow air to be drawn back into the system. Then move on to the next brake in the sequence until all have been bled.

A variation on manual bleeding is called "surge" bleeding. This procedure is sometimes needed to remove trapped air pockets in wheel calipers and wheel cylinders. The basic idea with this technique is to surge the fluid through the system so it will entrap the air. You'll need a helper because it's a two-person job. You start by having your helper pump the brake very hard and rapidly for about ten strokes. As he finishes the last stroke and holds the pedal down, you immediately open the bleeder screw and let the fluid escape while your helper maintains pressure. You then close the bleeder screw and repeat until there are no bubbles in the fluid.

Master cylinders can sometimes be tricky to bleed because air can become trapped in the bore, especially in vehicles where the master cylinder is mounted at an upward angle with respect to the firewall. Raising the rear of the car with a jack so the master cylinder is level can prevent air from being trapped in the front of the master cylinder while it is being bled. If the master cylinder is being replaced, it should always be "bench bled" off the car before it is installed. This is done by filling the fluid reservoir and attaching return hoses from the outlet ports to recycle the fluid back into the reservoir as the push rod is stroked.

On some vehicles with antilock brakes, a special procedure must be followed to remove air from the ABS hydraulic modulator. Those that require a special procedure bleeding typically usually need a "scan tool" to cycle the ABS solenoids in the modulator. The scan tool is plugged into the vehicle's onboard diagnostic connector to operate the solenoids. Scan tools are pretty expensive, so if your vehicle requires this type of bleeding procedure you may have to take it to a professional.
* Gravity bleeding -- This is a method of bleeding the brakes that allows fluid to dribble out of the open bleeder screws by gravity. It's a slow process and rarely used except in some import applications that require it because of metering valve arrangements that prevent normal manual or power bleeding procedures.
* Pressure bleeding -- The preferred technique used by most professional automotive technicians because it's the fastest way of getting the job done. Pressure bleeding (also called "power bleeding") uses a pressurized tank of fluid to force the old fluid out of the system. The pressure bleeding equipment is filled with the appropriate fluid, attached to the master cylinder reservoir with an adapter and pressurized with shop air. When the brake bleeder screws are opened, the fluid is forced through the lines. On some brake systems with metering valves, the metering valve must be held open so the pressurized fluid will pass through the valve. Special tools and clips are available for this purpose.
* Vacuum bleeding -- A less common but equally effective technique for bleeding the brakes. This process requires a special vacuum tool that used shop air to create a vacuum siphon to pull fluid through the lines and out of the bleeder screws. Vacuum bleeding does not require special adapters for the master cylinder nor do metering valves have to be held open.

The sequence or order in which the individual brakes are bled is very important to get all the air out of the system. The sequence varies from one vehicle to another, so always refer to the bleeding sequence specified by your vehicle manufacturer. You'll find this information in a shop manual.

The most common procedure on hydraulic systems that are split front-to-rear (most rear-wheel drive cars and trucks) is to do the rear brakes first, then the fronts, starting with the wheel furthest from the master cylinder. With systems that are split diagonally (most front-wheel drive cars and minivans), the sequence is often right rear, left front, then left rear and right front

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