Chap77

[object Object],[object Object],[object Object],[object Object],[object Object],OBJECTIVES: After studying Chapter 77, the reader should be able to: Continued

[object Object],[object Object],[object Object],KEY TERMS: Continued

DISC BRAKE DIAGNOSTIC PROCEDURE ,[object Object],Continued CAUTION: Do not test-drive the vehicle on public roads if the red brake warning light is on. After verifying the customer concern, the brake system should be carefully inspected and all parts and systems checked for proper operation.

[object Object],Continued ,[object Object],[object Object],[object Object]

[object Object],Step #3 Remove disc brake calipers and check the disc brakepads for proper lining thickness and check for cracks or other damage. Step #4 Replace all components that do not meet factory specs. Step #5 Test-drive the vehicle to verify that the repairs did correct the customer concern. Let The Owner Drive

VISUAL INSPECTION ,[object Object],Continued Figure 77–1 Minimum thickness for various types of disc brake pads. Disc brake pads can, of course, be replaced before they wear down to the factory-recommended minimum thickness.

[object Object],Figure 77–2 This cracked disc brake pad must be replaced even though it is thicker than the minimum allowed by the vehicle manufacturer. Continued

[object Object],Continued NOTE: Some disc brake pads use a heat barrier (thermo) layer between the steel backing plate and the friction material. The purpose of the heat barrier is to prevent heat from transferring into the caliper piston where it may cause the brake fluid to boil. Do not confuse the thickness of the barrier as part of the thickness of the friction lining material. The barrier material is usually a different color and usually can be distinguished from the lining material.

Figure 77–3 Be careful to observe the direction in which replacement linings are facing. Some vehicle manufacturers offset the friction material on the steel backing to help prevent or minimize tapered pad wear. Continued

Figure 77–4 Most disc brake calipers have a brake inspection opening. For a thorough inspection, however, the caliper should be removed and the entire braking system thoroughly inspected. ,[object Object],Be certain to position the pads correctly or severe tapered pad wear will occur.

[object Object],The Bleed and squirt Test

DISC BRAKE CALIPER SERVICE ,[object Object],Continued Figure 77–5 Both rear- and forward-mounted calipers have the bleeder valve at the top. Some calipers will fit on the wrong side of the vehicle, yet not be able to be bled correctly because the bleeder valve would point down, allowing trapped air to remain inside the caliper bore. If both calipers are being removed at the same time, mark them “left” and “right.” Knowing whether the caliper is “ rear mount ” position or “ forward mount” position is often needed when purchasing replacement calipers.

Figure 77–6 Many manufacturers recommend removing one-half of the brake fluid from the master cylinder before servicing disc brakes. Use a squeeze bulb and dispose of the used brake fluid correctly. ,[object Object],Continued

Figure 77–7 Most manufacturers recommend that the bleeder valve be opened and the brake fluid forced into a container rather than back into the master cylinder reservoir. This helps prevent contaminated brake fluid from being forced into the master cylinder where the dirt and contamination could cause problems. Continued

Figure 77–8 Many calipers use a hollow “banjo bolt” to retain the flexible brake line to the caliper housing. The fitting is usually round like a banjo. The copper washers should always be replaced and not reused. Continued

Figure 77–9 Caliper retaining bolts are often called guide pins. These guide pins are used to retain the caliper to the steering knuckle. These pins also slide through metal bushings and rubber O-rings. (Courtesy of EIS Brake Parts) Continued

Figure 77–10 If the caliper is not being removed, it must be supported properly so that the weight of the caliper is not pulling on the flexible rubber brake line. A suitable piece of wire, such as a coat hanger, may be used. Continued

Figure 77–11 A wooden block or a folded shop cloth helps prevent damage when caliper pistons are removed. ,[object Object],Continued

Figure 77–12 After the piston is removed from the caliper housing, the dust boot can often be removed using a straight-blade screwdriver. Continued

Figure 77–13 Phenolic (plastic) pistons should be carefully inspected. ,[object Object],Continued Since the mid-80s, phenolic caliper pistons have been used as original equipment by many vehicle manufacturers. They are natural insulators and keep heat from transferring to the brake fluid. Phenolic brake caliper pistons are lighter in weight than steel caliper pistons and are usually brown in color.

Figure 77–14 (a) The outside surface of caliper pistons should be carefully inspected. The square-cut O-ring inside the caliper rides on this outside surface of the piston. Sometimes dirty pistons can be cleaned and reused. (b) If there are any surface flaws such as rust pits on the piston, it should be replaced. ,[object Object],The stamped steel pistons are plated first with nickel, then chrome to achieve the desired surface finish. Continued

[object Object],Continued NOTE: Care should be taken when cleaning steel pistons. Use crocus cloth to remove any surface staining. Do not use sandpaper, emery cloth, or any other substance that may damage the chrome surface finish.

[object Object],Three Brake Jobs in 40,000 Miles - Part 1 Figure 77–15 These pads were found to be cracked and a section was missing from a part of one pad.

[object Object],Three Brake Jobs in 40,000 Miles - Part 2 The tech received permission to replace the rear brakes, overhaul both front calipers, and install quality disc brake pads. When the customer returned, the tech advised the customer to use the transmission on long downhill roads to help keep the brakes from overheating and failing prematurely. CAUTION: Installing disc brake pads on the wrong side of the vehicle (left versus right) will often prevent the sensor from making noise when the pads are worn down.

REASSEMBLING DISC BRAKE CALIPERS ,[object Object],Figure 77–16 Removing the square-cut O-ring seal from the caliper bore. Use a wooden or plastic tool to prevent damage to the seal groove. Continued Some manufacturers do not recommend honing the caliper bore because the actual sealing surface is between the piston seal and the piston itself.

Figure 77–17 Some manufacturers recommend cleaning the inside of the caliper bore using a honing tool as shown. Even though the caliper piston does not contact the inside of this bore, removing any surface rust or corrosion is important to prevent future problems. If the honing process cannot remove any pits or scored areas, the caliper should be replaced. ,[object Object],Continued

Figure 77–18 Installing a new piston seal. Never reuse old rubber parts. Step #1 Carefully clean the caliper bore with clean brake fluid from a sealed container. Coat a new piston seal with clean brake fluid and install it in the groove inside the caliper bore To assemble the disc brake caliper, perform the following: Continued

Figure 77–19 Brake assembly fluid or clean brake fluid from a sealed container can be used to lubricate the caliper seal and caliper pistons before assembly. ,[object Object],Step #3 Coat a new piston boot with brake fluid or brake assembly fluid . Brake assembly fluid is similar to brake fluid but has antiwear additives. ,[object Object],[object Object],Continued

Figure 77–20 Installing the caliper piston. Many calipers require that the dust boot be installed in the groove of the piston and/or caliper before installing the piston. ,[object Object],Figure 77–21 Installing a piston into a caliper. Sometimes a C-clamp is needed to install the piston. Both the piston and the piston seal should be coated in clean brake fluid before assembly.

Figure 77–22 Seating the dust boot into the caliper housing using a special plastic seating tool. ,[object Object],Continued

[object Object],Figure 77–23 All rubber bushings should be lubricated with silicone brake grease for proper operation. Continued

Figure 77–24 (a) Using a screwdriver to force the outboard pad into proper position before bending the retaining tabs. ,[object Object],Continued

Figure 77–24 (b) Use two hammers to bend the tab where it extends through the hole in the caliper body. Continued

Figure 77–25 Often, a hammer is necessary to bend the retainer flange to make certain that the pads fit tightly to the caliper. If the pads are loose, a “click” may be heard every time the brakes are depressed. This click occurs when the pad(s) move and then hit the caliper or caliper mount. If the pads are loose, a clicking noise may be heard while driving over rough road surfaces. Continued

[object Object],Using “Loaded” Calipers Saves Time Figure 77–26 A loaded caliper includes all hardware and shims with the correct pads all in one convenient package, ready to install on the vehicle. To save time (and sometimes money), many techs are simply replacing the old used calipers with “ loaded calipers ”, remanufactured calipers that include (come loaded) with the correct replacement friction pads and all necessary hardware.Only one part number is needed for each side of the vehicle for a complete disc brake overhaul.

Figure 77–27 Floating calipers must be able to slide during normal operation. Therefore, there must be clearance between the caliper and the caliper mounting pads (abutments). Too little clearance will prevent the caliper from sliding and too much clearance will cause the caliper to make a clunking noise when the brakes are applied. ,[object Object],The places where the caliper contacts the caliper mount are called the abutments , reaction pads , or ways . The sliding surfaces of the caliper support should be cleaned with a wire brush and coated with a synthetic grease or antiseize compound. See Figure 77-28. Continued

Figure 77–28 (a) Using an air-powered sanding disc to clean the caliper mount pads. (b) Applying silicone brake grease to the slides assures proper caliper operation. Continued (a) (b)

[object Object],Method 1 Replace the entire steering knuckle or caliper mount. This is the recommended and most expensive method. Replacements may be difficult to locate. Method 2 Some aftermarket brake supply companies offer “abutment repair kits” that include oversize slides.

ROTATING PISTONS BACK INTO THE CALIPER ,[object Object],Continued

[object Object],Continued NOTE: Some pistons are activated with left-handed threads.

Figure 77–29 Determine which face of the special tool best fits the holes or slots in the piston. Sometimes needle-nose pliers can be used to rotate the piston back into the caliper bore. Continued

[object Object],CAUTION: Never allow a customer to be the first to test-drive the vehicle after brake work has been performed.

[object Object],Always Double-Check Your Work Figure 77–30 Note the twisted flexible brake line. This can cause brake hose failure if not corrected.

[object Object],Foot Power is Stronger Than Air Pressure If compressed air has been tried with the caliper off the vehicle, reattach the caliper to the brake line. Bleed the brakes by opening the bleeder valve as normal to rid the caliper of any trapped air. Use a wood board or shop rag in front of the caliper piston and apply the brake. Pump the brake pedal as necessary to force the piston out. Some tech will not try to overhaul a caliper, but simply replace it, if air pressure will not remove the piston. CAUTION: Pump the brake pedal slowly. The piston normally will be gently pushed from the caliper. If the pedal is pushed hard, the caliper piston could be forced out of the caliper bore with tremendous force. Position the caliper so no damage will be done when the piston pops free.

DISC BRAKE SQUEAL ,[object Object],Continued Correcting Disc Brake Squeal Brake squeal can best be prevented by careful attention to details when servicing any disc brake. Some of these precautions include the following. Keeping the Disc Brake Pads Clean Grease on brake lining material causes the friction surface to be uneven. When brakes are applied, the uneven surface causes brake components to move.

[object Object],Figure 77–31 For best braking performance, purchase replacement disc brake pads that include all clips and shims specified by the vehicle manufacturer. Some pads come with a package of the specified grease to use on the shims to reduce the possibility of brake noise. Continued Original equipment brake pads use constrained layer shims ( CLS ) on the brake pads. These shims are constructed with dampening material between two layers of steel.

Continued NOTE: Many aftermarket disc brake pads do not include replacement hardware that usually includes noise-reducing shims and clips. One of the advantages of purchasing original equipment (OE) disc brake pads is that they usually come equipped with all necessary shims and often with special grease that is recommended to be used on metal shims.

[object Object],Continued 1. Lithium - based brake grease 2. Silicone grease 3. Molybdenum disulfide ( MOS 2 ) grease (“Molykote”) often referred to as “moly” grease 4. Synthetic grease (usually polyalphaolefin [ PAO ]) sometimes mixed with graphite, Teflon, and/or MOS 2 5. Antiseize compound The grease should be applied on both sides of shims used between the pad and the caliper piston.

NOTE: All metal-to-metal contacts must be lubricated to help prevent brake noise. CAUTION: Grease should only be applied to the nonfriction (steel) side of the disc brake pads. Continued

[object Object],Continued See the chart on Page 933 of your textbook.

Figure 77–32 Notice the beveled pads. The shape of the pad helps determine the frequency of any vibration in the brakes. Continued

DISC BRAKE SYMPTOM GUIDE ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Continued Pulls to One Side during Braking

[object Object],[object Object],[object Object],[object Object],[object Object],Continued Brake Roughness or Chatter (Pedal Pulsates)

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Continued Excessive Pedal Effort

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Continued Excessive Pedal Travel

[object Object],[object Object],[object Object],[object Object],[object Object],Continued Dragging Brakes

[object Object],[object Object],[object Object],[object Object],Continued Front Disc Brakes Very Sensitive to Light Brake Applications ,[object Object],[object Object],[object Object],Rear Drum Brakes Skidding under Hard Brake Applications

[object Object],The Screwdriver Trick - Part 1 A common trick used on the vehicle assembly line is to use a screwdriver to hold the piston against the rotor while an assistant releases the brake pedal. As the brake pedal is released, the adjusting screw inside the caliper piston is free to move. Sometimes, it may be necessary to tap on the caliper itself with a dead- blow hammer to free the adjusting screw. Repeat as necessary until the proper brake pedal height returns. If this method does not work, replace the caliper assembly. Step #1 Have an assistant depress the brake pedal.

[object Object],[object Object],[object Object],… Trick - Part 2 Figure 77–33 The screwdriver blade is used to keep the piston applied to allow self adjustment to occur when the brake pedal is released.

[object Object],Pressure Testing Can Help Find Problems Figure 77–34 (a) A brake pressure tester. (b) The small “pads” can be placed between the caliper piston and the rotor to check for applied pressure and inserted between the caliper and the rotor on the outside of the rotor to test the pressure—the pressure should be the same if the caliper is able to slide on its pins or slides. (a) (b)

[object Object],( cont. ) Continued

SUMMARY ,[object Object],[object Object],[object Object],Continued

SUMMARY ,[object Object],( cont. )

Chap77

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Chap77