Perhaps the toughest assignment for an auto parts professional is dealing with a warranty return on brake friction because, according to my experience, brake friction complaints generally revolve around brake squeal, brake pull, pedal pulsation, pedal response and service life issues. Occasionally, a brake friction manufacturer encounters complaints that develop during specific and often unexpected driving conditions. In those cases, however, brake engineers go to work to resolve these issues by improving their brake friction compounds. Unfortunately, most brake friction complaints result from incorrect application or installation errors. To help sort out brake friction complaints, let’s start by asking your technician or retail customer at least one of the following five questions:
WHAT IS THE APPLICATION?
The days of “good, better, or best” classifications of friction material has long past because application-specific friction materials should now be used to replace the original brake friction. Keep in mind that, qualitatively speaking, when we speak of “good” or “better,” we’re certainly not talking about the “best.” To illustrate the meaning of application-specific, there’s a big difference between the brake friction used on Grandma’s 1968 Plymouth Valiant and the brake friction used on a one-ton commercial delivery van. Grandma, who drives only to church on Sunday, wants smooth, quiet brakes. In contrast, the package delivery company wants long-lasting brake friction with aggressive stopping characteristics. It matters little if the brakes squeal or require slightly more pedal pressure than the formulation used in Grandma’s brake linings.
In today’s market, the general classifications of organic, semi-metallic and ceramic friction materials have tended to give way to a variety of friction materials designed to perform in a specific application under specific driving conditions. Consequently, it’s important to determine the type of vehicle being serviced, how it is being used, how much load it carries or tows and under what kind of driving conditions it is being driven. The brake manufacturer’s technical hotline can be also used to gather friction material and installation recommendations for specific applications.
Of course, application also begs the question of the brakes being abused. Always keep in mind that no brake friction can withstand abuse. When towing heavy loads downhill, drivers should always engage an intermediate gear range to relieve the load on the brakes and also operate the vehicle in a safe, sane manner. A good illustration of application and abuse might be a three-quarter ton truck being used to deliver building materials around town and the same type of three-quarter ton truck being used to tow a fully loaded horse trailer over high-mountain passes. Quite obviously, the truck used for delivery hardly warms the brakes while the truck used for towing more often than not turns the rotors red-hot due to the driver not using his intermediate gears for engine braking.
WHAT IS THE CONDITION OF THE ROTORS?
Because brake rotors form the foundation of a disk brake system, the rotor’s friction surface must be restored to like-new condition by replacement or resurfacing. While some manufacturers recommend re-using brake rotors that otherwise meet runout and thickness variation specifications, only a small percentage of brake rotors are really worth that risk. In fact, very few brake technicians are willing to risk a comeback by installing new pads on as-is brake rotors.
Many technicians also feel it’s more cost-effective to replace brake rotors because improperly resurfacing a brake rotor can cause more harm than good. If a rotor is being resurfaced, it’s important to understand that brake lathe integrity is vital to precision machining. Brake lathe arbor runout should be less than .001-inch and the bearing adapters, centering cones and holding fixtures should be free of grease, chips and dents. Cutting bits should be sharp and a small shop vacuum and hand brush should be available to clean chips away from rotor mounting surfaces.
Rotor condition is also important to the resurface/replacement decision. The rotor should have at least .020” of excess stock per side or .040” over minimum thickness specifications to accommodate machining. If the rotor surface is blued like a gun barrel, the rotors have been overheated through abuse and likely can’t be machined. Before the rotor is mounted on the lathe, the hub surfaces must be cleaned with a powered scuffing pad. Once mounted, the rotor should also be scratch-tested with the lathe’s cutting bit to ensure that the rotor is positioned on the lathe with the least runout.
Whether the rotor is machined on or off vehicle, most machining methods leave microscopically torn metal on the surface of the rotor. This torn metal should be removed before it can cause brake squeal by embedding in the surface of the new brake pads. Torn metal can be removed by sanding with 120/150 grit sand paper or with scuff pads designed for the purpose. Once sanding is completed, the resulting metal dust should be removed by washing in soap and water and immediately drying with a shop towel. Last, the wheel lug nuts should be torqued in an incremental, star pattern to prevent warping the new rotor.
HOW DO THE CALIPERS LOOK?
Technicians often forget that the caliper does all of the heavy lifting when it comes to keeping the brake pads parallel to the rotor and retracting the pads when the brake pedal is released. Indications of worn caliper guides or stuck caliper pistons include unequally worn brake pads, brake drag, suspension rattle and brake squeal.
When a brake pad is forced against the brake rotor to slow the vehicle, the pad tends to grab and release the rotor, which causes a series of high-frequency vibrations to occur at the pad. Brake calipers are designed to dampen this high-frequency pad vibration by using brake pad shims as a first line of defense to insulate the pad from the caliper. The next line of defense against brake noise are the silicone rubber grommets or sleeves used to insulate the caliper from the caliper mount. When these shims and grommets are worn or missing, high-frequency pad vibrations are then transmitted to, and magnified by, the suspension and chassis components.
The key point in resolving caliper issues is to replace calipers with cracked piston seals and sticking pistons. In addition, the shims and grommets supplied with the brake pads should be installed and lubricated with the silicone-based, high-temperature brake part lubricant included in the pad kit.
WHAT IS THE CONDITION OF THE REAR BRAKES?
Too many technicians, particularly Saturday afternoon technicians, forget that all four brakes should be in good working condition. On disc/drum brake configurations, the rear drum brakes generally last about twice as long as front disc brakes. As the rear drums wear, the front disc brakes then begin doing most of the braking, which results in accelerated pad wear and other brake friction complaints. This process happens because technicians often forget that drum brakes become inoperative due to rust or wear in their self-adjusting mechanisms.
In addition, some models of vehicles use the parking brake to adjust excess clearance out of the rear disc brake mechanism. Without the parking brake being used regularly, the rear disc brakes gradually become inoperative. To check rear brake adjustment, simply apply the parking brake and check brake pedal height. If normal brake pedal height returns by applying the park brake, the self-adjustment mechanism on the rear brakes is inoperative.
Replacing self adjustment hardware and return springs on the rear drums is a highly recommended procedure. If the vehicle has rear disc brakes, the parking brake mechanism should be inspected to ensure that it corrects for pad wear.
WHAT IS THE CONDITION OF THE SUSPENSION?
All too often, technicians forget that worn front suspension parts can cause huge deviations in wheel alignment angles when the brakes are applied. When the brakes are applied, loose suspension components often cause a brake-pulling condition accompanied by a clunking noise. When solving a brake performance issue, remember that all of the vehicle components must work together. When they don’t, the result can be another tough brake friction performance complaint.