A thorough inspection and evaluation is essential when it comes to replacing disc brakes on your vehicle. It is well worth the time it takes to remove the wheels and inspect the condition of your brake components before purchasing any parts. Before removing the wheels, make note of any noises, vibrations or harshness that occur during braking. What are the environmental conditions? At what speed does this occur? Does it happen during heavy or light braking? Keep in mind that not all problems experienced during braking are related to the brakes. For instance a clunk when you first step on the brake could indicate that the control arm bushings are worn and need to be replaced. A roaring noise that changes with the vehicle speed could be a faulty wheel bearing. Be sure to examine the brake components thoroughly. It is critical that all the components of the brake system are inspected for potential safety issues. If you’re not confident in your ability to do this, seek advice from a professional. This is a basic set of guidelines for common issues and procedures when dealing with disc brakes, and is by no means a comprehensive guide to diagnosing or replacing disc brakes.
The majority of brake concerns are related to noises and pulsation. Noisy brakes can be irritating at best, and could indicate that there is a safety concern.
Squealing: Disc brake squeal is heard when the brake pad vibrates at a high frequency. This vibration is usually caused by irregularities in the brake rotor that cause the brake pads to oscillate inside of their mounting points in the brake caliper assembly. The brake rotor tends to amplify the vibrations, acting as a speaker. The larger the gap between these parts, the more potential they have to make noise. Brake pad material and design can have an influence on brake squeal, as well as brake rotor material and design. Brake pads that are worn down to the wear indicators will cause squeals due to the design of the wear indicators themselves.
Grinding: Grinding noises usua3f74lly indicate a serious issue with the brakes, and should be addressed immediately. Grinding noises usually occur when the brake pad has worn down to the metal backing, and is contacting the rotor. This metal on metal wear generates extreme heat and wear on the pads and rotors. If the wear occurs for long enough, the caliper piston can pop out of the caliper and cause fluid loss leading to a complete loss of braking. In some cases the rotor can wear all the way to the vents and cause the wheel to lock up. Grinding noises could also indicate that the caliper mounting fasteners have come loose and the caliper itself is grinding up against the wheel or brake rotor.
Scraping: Scraping noises usually come from a thin metal contacting the rotor. Usually scraping noises are due to a dust shield that has been bent a little during service and is rubbing on the inside edge of the brake rotor. Scraping noises can also come from improper brake hardware installation, loose shims, or incorrect brake hardware contacting the brake rotor.
Clicking, Popping, or Clunking: These types of noises could be coming from the suspension due to a load change during braking, indicating worn bushings or ball joints. They can also come from the brake pad being too loose in the caliper bracket due to missing hardware, incorrectly sized brake pads, or the caliper not being secured correctly. Popping or clunking noises should be taken seriously, as they may indicate a potential safety concern.
Pulsation: Brake pulsation coming from the rear wheels will usually be felt in the seat, while brake pulsation coming from the front wheels will usually be felt in the steering wheel. Brake pulsation is caused by thickness variations in the brake rotor, and in some cases, a warped rotor or bent wheel hub. Rotors can become warped due to improper or uneven wheel torque, rust buildup on the wheel hub, or inadequate heat dissipation. Brake rotor runout can be checked with a dial indicator. Check the manufacturer’s specifications runout and minimum thickness.
Grabbing: Brake grabbing is usually due to contamination from oil or grease that has soaked into the friction material, producing a jerky braking experience. This can come from oil leaks, axles slinging out grease, and in some cases tire or wheel shine compounds that have come in contact with the brake surfaces during application. The source of contamination must be located and remedied, and the brakes must be replaced.
Dragging: Brake dragging occurs when the brake caliper is not able to retract from the applied position and may cause the vehicle to pull to one side. Dragging may lead to brake fade. This may occur due to a stuck caliper piston, stuck caliper slides, restricted brake hoses, or incorrectly installed brake parts.
Fading: Brake fade is a temporary loss of braking performance due to excessive heat build-up. Brake fade occurs when the compounds in the friction material are heated to a point which they begin to change states from solid to gas. This gas begins to form a boundary between the friction material and the rotor, and cause the brake pad to be pushed away from the disc. This is most common with new brake pads, as the bonding agents in the pad have not yet been heated. Another type of brake fade occurs when the brake fluid has degraded and has absorbed a significant amount of moisture due to its hydroscopic nature. Moisture content in the brake fluid lowers its boiling point. The brake fluid begins to vaporize and acts like air in the brake lines, requiring extra compression to exert force on the brakes, causing reduced braking performance, and potentially a complete loss of braking ability. Braking ability is restored when the fluid cools below the boiling point. Brake fluid can be checked with a refractometer or test strips which indicate the amount of moisture, boiling point, and other compounds which may be present.
Spongy, Soft, or Low Pedal: A brake pedal that feels spongy when depressed indicates that there is air in the hydraulic system. Pumping the brake pedal several times will temporarily restore the pedal. A soft or low pedal can be due to a brake fluid leak or a faulty master cylinder. In most cases of faulty master cylinders, depressing the brake pedal quickly with high force will sometimes temporarily restore the brake pedal. This is due to worn cup seals inside the master cylinder which are allowed to expand under rapid high force.
Hard Pedal: A brake pedal that requires excessive force to apply the brake is caused by lack of brake boost. Brake boost is achieved by means of a vacuum booster, or a hydraulic booster depending on the vehicle. In vacuum boosted systems, vacuum usually comes from the intake manifold, and is maintained by the use of a one way check valve. In situations where manifold vacuum is not available, such as vehicles with diesel engines, a vacuum pump is used. Check for vacuum leaks in the main line between the intake manifold or vacuum pump to the brake booster. Hydraulic systems require a pump, which pressurizes fluid to achieve brake boost, and an accumulator to retain pressure for multiple applications of the brakes. Check for any leaks, verify that the pump is building pressure, and make sure the accumulator is able to retain the pressure.
There are an overwhelming amount of options available when choosing disk brakes for your vehicle. Becoming familiar with all of the options available will help you make the right choice when determining the parts that are right for your needs. It is important to consider what you want in terms of cost, performance, noise, and wear.
Brake Pads: There are several different friction materials available, and every brake manufacturer has their own recipe for what materials go into their brake pads. While there is a general idea of what friction materials make up a certain type of brake pad, there is a wide degree of variance between manufacturers. The friction materials must be able to absorb and dissipate heat efficiently. Brake pad design also plays a large role in performance and noise characteristics. Brake pads with chamfered edges cut down on noise, while brake pads with slotted surfaces help reduce brake fade. Knowing the basics can help you decide what is best for you.
- NAO (Non Asbestos Organic): Organic brake pads are made from natural fibers that are capable of withstanding high amounts of heat. They are generally quieter than any other type of brake pad. They are made of softer materials than other types of brake pads, due to this, they tend to wear quickly and leave a significant amount of dust behind. Due to their soft nature, organic pads require more pedal effort to compress the friction material, and also have a low resistance to brake fade. Reduced performance is also noted when organic pads become wet.
- Semi Metallic: Semi Metallic brake pads are comprised mostly of steel and other metallic compounds, and have a higher density than other types of brake pads. This allows for excellent performance at very high temperatures. Semi metallic pads have a high resistance to wear and brake fade; however, brake rotor wear is increased. Due to their composition of steel and various metals, semi metallic pads also tend to make more noise and dust than other types of brake pads.
- Ceramic: Ceramic brake pads are made up of ceramic and a number of various compounds, including copper fibers. Ceramic brake pads are able to handle high temperatures with a stable coefficient of friction over a wide temperature range. Ceramic brake pads offer a high resistance to brake fade, but not as much as semi metallic brake pads. Brake rotor wear, noise, and dusting are considerably lower with ceramic brake pads compared to their semi metallic counterparts. Due to the cost of manufacturing, ceramic pads are generally more expensive than others.
Brake Rotors: Brake rotors are the heart of any disc brake system. Brake rotors are not only the main component in converting the vehicle momentum into heat, they are also responsible for dissipating the heat produced by the friction of the brake pads. A brake rotor provides the surface area for the brake pad to act upon. The amount of heat dissipated by the rotor depends on the surface area and mass of the rotor, or the diameter and thickness of the rotor. This heat dissipation is often improved by venting, which helps force the heat away from the rotor. Some rotors may have a specific vent direction and may only be fit on a certain side of the vehicle.
- Vented/Non Vented: Brake rotors will be made up of either a solid disc, or two separate discs sandwiched together with a series of vents in the middle. The vents in a brake rotor allow it to force heat away by means of centrifugal force. Almost all front brake rotors are vented. Often times, rear brake rotors will be solid, especially on front wheel drive cars where most of the braking force is required at the front of the vehicle. Quality vented rotors will have thicker friction discs, whereas economy versions may provide less material, with a wider venting area.
- Slotted/Drilled: Gasses formed from excessive heating of the brake pads can form a barrier between the pad and rotor. Brake rotors with slots on the face provide a path for the gasses to escape, which reduces the potential for brake fade to occur. Drilling the rotors provides a greater escape point, preventing the potential for gas to form between the pad and the rotor. Drilling also aids in cooling the rotor.
- Composite: Brake rotors are usually made of solid cast iron, which has excellent heat absorption properties. Composite rotors are made of a stamped steel inner hat surface along with a cast iron friction surface. Composite rotors are designed to save weight and lower production cost. Some composite rotors may include directional vents for improved cooling, due to the lower mass of the center of the rotor. An improvement of this design is the floating rotor, which features a billet aluminum hat portion and a high carbon cast iron friction surface. The floating rotor helps reduce thermal stress and reduces the vehicle’s unsprung mass, which allows for improved handling and braking performance.
Always verify that the parts are correct prior to installation. This will insure a smooth installation and decrease the chance of having any issues after the job is done. Make sure you have all the tools, parts, and materials required before disassembly.
Brake Rotors: Check that the new brake rotors are the correct diameter and that the center hub mounting bore is also the correct diameter. Most rotors come with an oil film to prevent rust during shipping. This must be completely cleaned off with brake parts cleaner prior to installation. Some rotors have a zinc coating which should not be cleaned off. This coating helps prevent rust on the non-friction surfaces of the rotor. Check runout before and after replacing rotors. If a new rotor has excessive runout the warranty will usually be void if it has been used for any period of time.
Brake Pads: Check that the brake pads are the same size and that they have all the associated shims, wear indicators, and indexing pins as the old pads. Some systems will have a specific inner and outer pad which are oriented on a particular side of the vehicle. For example, if all pads appear to be the same but only two have wear indicators, the pads with the indicators will be the inside pads. The wear indicators should face so that they will be pushed toward the pad by the rotor when the vehicle is traveling in a forward direction. This will cause them to squeal when the pads are worn below minimum thickness.
Brake Hardware: It is recommended that you install new hardware when a complete brake job is performed. Disc brake hardware is made of spring steel and loses its retention when it is heated and cooled over several cycles. Check that all mounting hardware, pad spreaders, abutment clips, slide pins, and rubber boots are present and correct before beginning the job.
Brake Lubricant: Make sure you use a lubricant that is specifically designed for disc brakes. Disc brake lubricants are formulated to tolerate high temperature conditions, high force, brake dust, road salt and grime, and they will not be washed away by water. Disc brake lubricants are non-petroleum based and are comprised of materials such as molybdenum disulfide, silicon, and several other synthetic compounds. Molybdenum disulfide, or moly lubricants, are recommended for use on metal to metal points that require lubrication. Silicon based lubricants, and other lubricants are recommended for use on caliper slides and surfaces with rubber contact.
Tools and Supplies: Other than the usual sockets, wrenches, and screwdrivers needed to perform most repairs, you will need a few specific tools and supplies to ensure a proper brake job. Here are a few basic ones:
- Wire Brush: You will need a good wire brush to help clean the grime from the brake calipers and brackets
- Emery Cloth/Sandpaper: Along with a wire brush, this will help to remove any rust on the sliding surfaces of the caliper assembly, and also clean any rust of the wheel hub surface.
- C-Clamp/ D-Clamp Locking Pliers: These will help compress the caliper pistons on most calipers with ease, and without damage to the hardware.
- Hammer and punches: Sometimes a stuck brake rotor is almost impossible to remove without a little persuasion. Strike the rotor on the hub face with a hammer to free it up. Small punches help to remove the brake pad retaining pins on fixed calipers.
- Caliper Piston Compressor/Retractor: On rear brake calipers with an integral parking brake, the caliper piston will need to be retracted into the caliper by screwing it in and applying force at the same time. This tool will save a lot of time and frustration. On fixed calipers, a pad spreader can help make the job a little easier.
- Impact Driver: Many brake rotors are secured to the hub with a #3 Phillips head bolt. You’re most likely not going to be able to remove this with a screwdriver. An impact driver makes removing this bolt easy. Alternatively, a hammer and chisel can help shock it loose so it can be turned by hand
- 8X1.25mm Bolts: Some brake rotors have two threaded holes 180 degrees apart on the hub face. A couple of bolts can be inserted and tightened in lieu of using a hammer to help break the rotor free
- Brake Parts Cleaner: A few cans of brake parts cleaner will be necessary to clean the oil film from brake rotors as well as help clean grease and brake dust from the brake parts.
Clean and Lube: With the brakes disassembled and the caliper pistons retracted, clean the caliper and bracket thoroughly with a wire brush and brake parts cleaner. You want to remove all of the caked on brake dust. If the sliding surfaces that the brake pads or caliper come in contact with are rusty, burred, or uneven, recondition them with some emery cloth or sandpaper. Also, clean the face of the caliper piston and the caliper “fingers” where the backing of the brake pads rest against. You want a clean smooth surface for the moving parts to ride on. Clean the wheel hub face thoroughly to remove any rust present, then apply a thin coat of anti-seize compound to the hub face to prevent rust buildup between the wheel hub and brake rotor. A few thousandths of an inch of rust buildup here can multiply and cause an uneven brake rotor position in relation to the hub face and lead to pulsation down the road. Apply disc brake lube to any metal on metal surface where moving parts slide against each other, including the caliper pistons and “fingers” where the backing of the brake pads come into contact with. Clean the brake caliper slide pins and apply a thin coat of lube and ensure they move with minimal effort. Be sure not to get any lube on the friction surfaces.
Hardware: Make sure that all hardware is present and that it is clean and secure in its mountings. Loose hardware can cause squeals and possibly damage to the brake components if it is worn or installed correctly. Check for proper fit and movement, Verify that the pads are snug in their mounting points but allowed to move freely. Verify that the caliper slide pins move freely and the dust boots are in place and not damaged.
Torque: Tighten caliper and caliper bracket bolts to the recommended torque specification. It is extremely important to torque the wheel lugs in the proper pattern, to the proper torque specification. The brake rotors heat up and cool down in rapid cycles, sometimes almost instantly such as driving through a large puddle. It is imperative that the wheel lugs exert an even force on the brake rotor during these heating and cooling cycles to prevent warpage of the brake rotor. Always torque the wheel lugs when they are clean and dry. Use a wire brush to clean off any rust, oil, or debris. Replace wheel lugs that are damaged. Never apply anti-seize to wheel studs, because this can increase the torque and result in broken lugs or improperly torqued wheels.
Seat the Pads: Don’t forget to pump the brake pedal several times before starting the engine. This will allow the caliper pistons to squeeze the pads against the rotors before you begin driving. Now you are ready to begin the bedding in procedure. The new brake pads need to establish an even contact with the brake rotor. They will need to be heated up gradually so that they begin to to transfer a film onto the new or resurfaced brake rotor. It can take up 500 miles to establish an even transfer film, but the first few stops are the most important. If the pads get too hot, the bonding agents in the pad will be pulled to the friction surface and will cause the pad to glaze over. If the pads are glazed, they cannot establish a good transfer film. There are many schools of thought when it comes to the bedding in procedure, and each brake manufacturer has their own specific procedure for bedding in brakes. An example, and a good general rule is the 30-30-30. Perform 30 moderate stops from 30 miles per hour with 30 seconds between stops for cooling. Check with your manufacturer for their preferred method. Try to avoid hard braking for the first 500 miles to ensure a proper break in of your new brakes.
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