hub and rotor assembly
A ventilated rotor is cast with a weblike construction between two friction surfaces．The webs radiate out from the center of the rotor, much like vanes or fins in a fan . As the rotor turns，air is drawn into the rotor at its center , flows between the friction surfaces, and is discharged along the outer edge . this cools the rotor . which drastically reduces the chances of brake fade , even during multiple hard stops .
a splash shield protects the rotor and pads from road splashes and dirt . it is also shaped to channel the flow of air over the exposed rotor surfaces . as long as the car is moving , this flow of air helps to cool the rotor . the splash shield cannot be removed unless the rotor and caliper are first removed . replacement of the splash shield is necessary only when it has been damaged or when the spindle is replaced .
caliper assembly
a brake caliper converts hydraulic pressure into mechanical force . the caliper housing is usually a one-piece construction of cast iron or aluminum and has an inspection hole in the top to allow for lining wear inspection . the housing contains the cylinder bore . in the cylinder bore is a groove that seats a square-cut seal . this groove is tapered toward the bottom of the bore to increase the compression on the edge of the bore to increase the compression on the edge of the seal that is nearest hydraulic pressure . the top of the cylinder bore is also grooved as a seat
for the dust boot . a fluid inlet hole is machined into the bottom of the cylinder bore and a bleeder valve is located near the top of the casting .
a caliper can contain one , two , or four cylinder bores and pistons that provide uniform pressure distribution against the brake's friction pads . the pistons are relatively large in diameter and short in stroke to provide high pressure on the friction pad assemblies with a minimum of fluid displacement .
basically the hydraulics of disc brakes are the same as for drum brakes , in that master cylinder piston forces the brake fluid into the wheel cylinders and against the wheel pistons .
the disc brake piston is made of steel , aluminum , or fiberglass-reinforced phenolic resin . steel pistons are usually nickel-chrome plated for improved durability and smoothness . the top of the pistons is grooved to accept the dust boot that seats in a groove at the top of the cylinder bore and also in a groove in the piston . the dust boot prevents moisture and road contamination from entering the bore .
a piston hydraulic seal prevents fluid leakage between the cylinder bore wall and the piston . this rubber sealing ring also acts as a retracting mechanism for the piston when hydraulic pressure is released , causing the piston to return in its bore (Figure 46-3). when hydraulic pressure is diminished , the seal functions as a return spring to retract the piston .
In addition，as the disc brake pads wear , the seal allows the piston to move farther out to adjust automatically for the wear , without allowing fluid to leak．Since the brake pads need to retract only slightly after they have been applied，the piston moves back only slightly into its bore．The additional brake fluid in the caliper bore keeps the piston out and ready to clamp the surface of the rotor．
fixed caliper disc brakes
fixed caliper disc brakes have a caliper assembly that is bolted in a fixed position and does not move when the brakes are applied . the pistons in both sides of the caliper come inward to force the pads against the rotor (Figure 46-4).
floating caliper disc brakes
a typical floating caliper disc brake is a one-piece casting that has one hydraulic cylinder and a single piston . the caliper is attached to the spindle anchor plate with two threaded locating pins . a Teflon sleeve separates the caliper housing from each pin and the caliper slides back and forth on the pins as the brakes are actuated . when the brakes are applied , hydraulic pressure builds in the cylinder behind the piston and seal . because hydraulic pressure exerts equal force in all directions , the piston moves evenly out of its bore .
the piston presses the inboard pad against the rotor . as the pad contacts the revolving rotor , greater resistance to outward movement is increased , forcing
pressure to push the caliper away from the piston , this action forces the outboard pad against the rotor (figure 46-5). however , both pads are applied with equal pressure .
sliding caliper disc brakes
With a sliding caliper assembly , the caliper slides or moves sideways when the brakes are applied．As mentioned previously ,in operation，these brakes are almost identical to the floating type．But unlike the floating caliper ,the sliding caliper does not float on pins or bolts attached to the anchor plate (Figure 46-6)．It has angular machined surfaces at each end that slide in mating machined surfaces on the anchor plate．This is where the caliper slides back and forth .
Some sliding calipers use a support key to locate and support the caliper in the anchor plate . the caliper support key is inserted between the caliper and the anchor plate . a worn support key may cause tapered brake pad wear . always inspect the support keys when replacing brake pads . also make sure they are lubricated when reassembling the unit .
brake pad assembly
brake pads are metal plates with the linings either riveted or bonded to them . pads are placed at each side of the caliper and straddle the rotor . the inner brake pad , which is positioned against the piston , is not inter changeable with the outer brake pad . the linings are made of semimetallic or other nonasbestos material
(Figure 46-7).
DISC PAD WEAR SENSORS
Some brake shoe pads have wear sensing indicators．The three most common design weal sensors are audible，visual，and tactile .
Audible sensors are thin，spring steel tabs that are riveted to or installed onto the edge of the pad's backing plate and are bent to contact the rotor when the lining wears down to a point that replacement is necessary ．At that point，the sensor causes a high-pitched squeal whenever the wheel is turning，except when the brakes are applied，then the noise goes away．The noise gives a warning to the driver that brake service is needed and perhaps saves the rotor from destruction (Figure 46-8). the tab is generally installed toward the rear of the wheel .
visual sensors inform the driver of the need for new linings . this method employs electrical contacts recessed in the pads that touch the rotor when the linings are worn out . this completes a circuit and turns on a dashboard warning light . this system is found mostly on imports .
tactile sensors create pedal pulsation as the sensor on the rotor face contracts the sensor attached to the lower portion of the disc pad .
caution
as disc brake pad linings wear thin , more brake fluid is needed in the system.
Customer care
Excessive heat liquefies the resin binder that holds the brake pad material together . once liquefied , the binder rises to the surface of the pad to form a glaze .
A glazed pad may cause squealing because more heat is needed to achieve an amount of friction equal to a pad in good condition．One common cause of pad glazing is the improper break-in of the pads．Remember to inform customers that unnecessary hard braking during the first 200 miles(320km)of a pad’s life can generate enough heat to glaze the pads and ruin the quality of the work just performed ．There are Pads now available that require no break-in time .
Rear Disc / Drum (Auxiliary Drum) Parking Brake
The rear disc / drum or auxiliary drum parking brake arrangement is found on some vehicles . on these brakes , the inside of each rear wheel hub and rotor assembly is used as the parking brake drum(Figure 46—9) . The auxiliary drum brake is a smaller version of a drum brake and is serviced 1ike any other drum brake .
Rear Disc Parking Brakes
Instead of using an auxiliary drum and shoes to hold the vehicle when parked，these brakes have a mechanism that forces the pads against the rotor mechanically．One
method
for
doing
this
is
the
ball-and-ramp
arrangement．Another method that is found on many vehicles has a threaded，spring-loaded pushrod (Figure 46-10)．As the parking brakes are applied ，a mechanism rotates or unscrews the pushrod ，which in turn pushes the piston out ．Other types of actuating systems for rear disc brakes include the use of a variety of cams．
DISC BRAKE DIAGNOSIS
Many problems that are experienced on vehicles with disc brakes are the same ones that are evident with drum brake systems ．There are some problems that occur only with disc brakes．Before covering the typical complaints , it is important to remind you to get as much information as possible about the complaint from the customer．Then road test the vehicle to verify the complaint．A complete inspection of the rotor , caliper , and pads(Figure 46-11)
should be done anytime you are working on the brakes．
What follows is a brief discussion of common complaints and their typical causes．
The red warning light indicates there is a problem in the regular brake system , such as low brake fluid levels or that the parking brake is on . a low fluid light may
be present in addition to the red brake warning light . whenever the fluid is low , you should suspect a leak or very worn brake pads .
The yellow or amber brake warning light is tied into the antilock brake system . this light turns on for two reason : the ABS system is performing a self-text or there is a fault in the ABS system .
A blue warning light lets the driver know the wheels are slipping because of poor road conditions . more discussion on the amber and blue lights is given in the next chapter of this book .
Pulsating pedal
Customers will feel a vibration or pulsation in the brake pedal when the brakes are applied if a brake rotor is warped . if this symptom exists , check the rotors for runout and parallelism . a warped rotor should be replaced and is often caused by improper tightening of the wheel lug nuts . in fact , uneven lug nut torque can cause a pulsating brake pedal . you should be aware that pedal pulsation is normal on vehicles with ABS when the antilock brake system is working .
Spongy Pedal
With a spongy pedal , the customer will probably feel the need to pump the brake pedal to get good stopping ability . the complaint may also be described as a soft pedal . this problem is caused by air in the hydraulic system . Although
bleeding the system may remove the air，you
should always question how the air got in there ．Check for leaks and for proper master cylinder operation．
Hard Pedal
The driver’s complaint of a hard pedal normally indicates a problem with the power brake booster . However it can also be caused by a restricted brake line or hose . Carefully check the lines and hoses for damage . Feel the brake hoses . if they seem to have lost their rigidity , the hose may have collapsed on the inside and this is causing the restriction . Restrictions can also be caused by frozen caliper or wheel cylinder pistons．
Dragging Brakes
Dragging brake make the vehicle feel as if it has lost or is losing power as it drives down the road . the problem also wastes a lot of fuel and generates destructive amounts of heat that can cause serious brake damage and brake failure . while trying to find the cause of this problem , check the parking brake first . make sure it is off . check the rear wheels to make sure the parking brakes are released when they should be . if the problem is not in the parking brakes , check for sticky or seized pistons at the calipers and wheel cylinders .
Grabbing Brakes
When the brakes seem to be overly sensitive to pedal pressure , they are grabbing . normally this problem is caused by contaminated brake linings . if the lings are covered or saturated with oil , find the source of the oil and repair it . then replace the pads and refinish or replace the rotor .
Noise
If the customer’s complaint is noisy brakes , verify during the road test that the problem is in the brakes . if the noise is caused by the brakes , pay attention to the type of noise and let that lead to the source of the problem . remember , some brake pads have wear sensors that are designed to make a high-pitched squeal when the pads are worn . other causes could be the rotor rubbing against the splash shield or that something has become wedged between the rotor and another part of the vehicle . noise may also be caused by failure to install all of the hardware when placing a caliper or brake pads in service .
Pulling
When a vehicle drifts or pulls to one side while cruising or when braking , the cause could be in the brake system or in the steering and suspension system . check the inflation of the tires , the tires’ tread condition , and verify that the tires on each axle are the same size . check the operation of the brakes . if only one front wheel is actually doing the braking , the vehicle will seem to stumble or pivot on that one wheel . if no problems are found in the brake system , suspect an alignment or suspension problem .
SERVICE PRECAUTIONS
The following general service precautions apply to all disc brake systems and should always be followed :
1. Be sure the vehicle is properly centered and secured on stands or a hoist .
2. If the vehicle has antilock brakes , depressurize the system according to the procedures given in the service manual .
3. Disconnect the battery ground cable .
4. Before any service is performed , carefully check the following :
■tires for excessive wear or improper inflation
■wheels for bent or warped rims
■wheel bearings for looseness or wear
■suspension components to see if they are worn or broken
■brake fluid level
■master cylinder , brake lines or hoses , and each wheel for leaks
5. During servicing , grease , oil , brake fluid , or any other foreign material must be kept ff the brake linings , caliper , surfaces of the disc , and external surfaces of the hub . handle the brake disc and caliper in such a way as to avoid deformation of the disc and nicking or scratching of the brake linings .
6. when a hydraulic hose is disconnected , plug it to prevent any foreign material from entering .
7. never permit the caliper assembly to hang with its weight on the brake hose . support it on the suspension or hang it by a piece of wire .
8. inspect the caliper for leaks . if leakage is present , the caliper must be over hauled .
9. when using compressed air to remove caliper pistons , avoid high pressures . a safe pressure to use is 30 psi (207 kPa).
10. clean the brake components in either denatured alcohol or clean brake fluid . do not use mineral-based cleaning solvent such as gasoline , kerosene , carbon tetrachloride , acetone , or paint thinner to clean the caliper . it causes rubber parts to become soft and swollen in an extremely short time .
11. lubricate any moving member such as the caliper housing or mounting bracket to assure a free-moving action . use only recommended lubricant .
12. before the brake pads are installed , apply a disc brake noise suppressor to the back of the pads to prevent brake squeal . for best results , follow the directions on the container .
13. obtain a firm brake pedal after servicing the brakes and before moving the vehicle . be sure to road test the vehicle .
14. always torque the lug nuts when installing a wheel on a vehicle with disc brakes . never use an impact gun to tighten the lug nuts . warpage of the rotor could result if an impact gun is used .
Before beginning brake work , remove about two-thirds of the brake fluid from the master cylinder’s reservoir . if this is not done , the fluid could overflow and spill when the pistons are forced back into the caliper bore , possibly damaging the painted surfaces . replace the cover . discard old brake fluid .
Another common procedure (and perhaps a better way) is to open the caliper bleeder screw and run a hose down to a container to catch the fluid that is expelled when the piston is forced back into its bore . this also makes it easier to move the piston .
If the bleeder screws are frozen tight with corrosion , it is sometimes possible to free them using a propane torch and penetrating oil . of course , the caliper has to be removed from the car , taken to a bench and worked on there．If the bleeder screws cannot be loosened，they can be drilled out and the caliper retapped for an
insert , or
the caliper can be replaced with a new or rebuilt unit . the bleeder screw should be removed when doing an overhaul .
warning
When using the propane torch to loosen a bleeder screw , use it with extreme care .
GENERAL CALIPER INSPECTING AND SERVICING
Frequently , caliper service involves only the removal and installation of the brake pads . however , since the new pads are thicker than the worn-out set they replace , they locate the piston farther back in the bore where dirt and corrosion might cause the seals to leak . for this reason , it is often good practice to carefully inspect the calipers whenever installing new pads . of course , it is also good practice to true-up or replace the rotors when replacing brake pads .
When bench working a caliper assembly , use a vise that is equipped with protector jaws . excessive vise pressure causes bore and piston distortion .
Caliper Removal
To be able to replace brake pads , service the rotor , or to replace the caliper , the caliper must be removed . the procedure for doing this will vary according to
caliper design . always follow the specific procedures given in a service manual . use the following as an example of these procedures :
1. remove the brake fluid from the master cylinder .
2. raise the vehicle and remove the wheel and tire assembly .
3. on a sliding or floating caliper , install a C-clamp with the solid end of the clamp on the caliper housing and the screw end on the metal portion of the outboard brake pad . tighten the clamp until the piston bottoms in the caliper bore (figure 46-12), then remove the clamp．Bottoming the piston allows room for the brake pad to slide over the ridge of rust that accumulates on the edge of the rotor．
4. On threaded-type rear calipers . the piston must be rotated to depress it．This requires a special tool (Figure 46-13).
5. Disconnect the brake hose from the caliper and remove the copper gasket or washer and cap the end of the brake hose．If only the brake pads are to be replaced，do not disconnect the brake hose．
6. Remove the two mounting brackets to the steering knuckle bolts ．Support the caliper when removing the second bolt to prevent the caliper from falling．
7. On a sliding caliper , remove the top bolts , retainer clip , and antirattle springs(Figure 46-14) . On a floating caliper , remove the two special pins that hold
the caliper to the anchor plate(Figure 46-15) . On a fixed caliper , remove the bolts holding it to the steering knuckle . On all three types , get the caliper off by prying it straight up and lifting it clear of the rotor .
Brake Pad Removal
Disc brake linings should be checked periodically or whenever the wheels are removed．Some calipers have inspection holes in the caliper body．If they do not，the brake pads can be visually inspected from the outer ends of the caliper .
If you are not sure the pads are worn enough to warrant replacement , measure them at the thinnest part of the pad . compare this measurement to the minimum brake pad lining thickness listed in the vehicle’s service manual , and replace the pads if needed .
When a pad on one side of the rotor has worn more than on the other side , the condition is called uneven wear . uneven pad wear of often means the caliper is sticking and not giving equal pressure to both pads . on a sliding caliper , the problem could be caused by poor lubrication of or deformation of the machined sliding areas on the caliper and/or anchor plate . a slightly tapered wear pattern on the pads of certain mode is caused by caliper twist during braking . it is normal if it does not exceed 1/8-inch (3-mm) taper from one end of the pad to the other (figure 46-16) .
Sliding or floating caliper must always be lifted off the rotor for pad
replacement . fixed calipers might have pads that can be replaced by removing the retaining pins or clips instead of having to lift off the entire caliper . brake pads may be held in position by retaining pins , guide pins , or a support key . note the position of the shims , anti-rattle clips , keys , bushings , or pins during disassembly . a typical procedure for replacing brake pads is outlined in Photo Sequence 50 , included in this chapter .
If only the pads are going to be replaced , lift the caliper off the rotor and hang it up by a wire . remove the outer pad and inner pad . remove the old sleeves and bushings and install new ones . replace rusty pins on a floating caliper to provide for free movement . transfer shoe retainers , which can be clips or springs , onto the new pads .
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