Pro-Cut International sponsored a gathering of brake experts from around the industry to talk about a variety of brake service and repair issues. Attendees include brake engineers from DaimlerChrysler, Ford and General Motors, several suppliers of brake service tools, representatives from a variety of different types of service outlets (retailers like PepBoys, tire chains, muffler shops, etc.), training instructors, people from AAA and the Motorist Assurance Program (MAP), plus members of the media.
The purpose of the symposium was to share knowledge and real world experience with our readers so you can better understand some of the brake repair issues on late model vehicles.
The hottest topics for discussion were:
* Brake rotors -- Should they or shouldn't they be resurfaced when doing a brake job? If so, what's the best way to resurface rotors?
* Brake fluid -- What do the OEMs say about inspecting and changing brake fluid for preventive maintenance?
* Calipers -- Should calipers be rebuilt or replaced when doing a brake job?
* Changes in OEM brake technology that are extending the service life of the brake system.
CHRYSLER BRAKE NOTES
ABS is currently available on all Chrysler/Dodge/Jeep products, but is standard only on the Jeep Grand Cherokee. On everything else, it's an extra cost option. Traction control is available on Chrysler cars only, but is coming to Dodge and Jeep trucks and SUVs. Stability Control (ESP) is standard on Mercedes only, but will be added to future Chrysler products as the OEM supplier (Bosch) develops the system for these applications.
Electro-Hydraulic Braking (EHB) is a "brake-by-wire" system that eliminates the physical connection between the brake pedal and brake hydraulics. The brake booster and master cylinder are replaced with a "pedal simulator" and failsafe hydraulic manifold to actuate the brakes through the EHB unit. Chrysler began offering brake-by-wire back in 2002 on certain luxury models. Further down the road, electro-mechanical braking that uses no hydraulics at all may find its way onto vehicles.
Mercedes has been using brake rotors made of ceramics on some of its models (such as the Porsche 91). The ceramic rotor is very lightweight and does not conduct heat into the hub, but it's also extremely expensive ($900 each!). It wears very little, but if it is damaged it can't be resurfaced and must be replaced. It also requires a special type of high temperature friction material to withstand the heat.
Mercedes is also using drilled and slotted rotors on some of its high performance models. The Chrysler engineers said they don't offer a significant improvement in cooling compared to a standard rotor, and are used primarily to enhance the "racing image" of the vehicle.
On the issue of brake life, premature pad wear and rotor wear are major customer issues with all the vehicle manufacturers. The latest approach is to make the design of the brake system more robust so the brakes last longer and run quieter. Chrysler tests for wear using city traffic driving runs. They load the vehicle with 400 lbs. of extra weight to simulate a heavy load, then drive the vehicle for 4,000 to 5,000 miles and measure the pads and rotor to see how much they're worn. The results are then extrapolated to estimate the service life of the brake components.
One engineer said a performance target for Jeep products is to have the front brakes last at least 25,000 miles and the rear brakes 50,000 miles in the city traffic driving test. Because the test assumes a "worst case" situation, most vehicle owners will probably get a lot more miles out of their brakes.
The Chrysler engineers said their design goal is to make the rear brakes last twice as long as the ones up front so the rear brake linings only have to be replaced at every other pad change. Right now, that isn't necessarily the case. On many Jeep Grand Cherokee models, the rear brakes use an aggressive friction material that wears the rear rotors and may require the rear rotors to be replaced when the pads are changed.
The current design life for OEM brake calipers is now 150,000 to 180,000 miles, says Chrysler. Some Jeep Cherokee owners have reported getting 200,000 to 300,000 miles out of their calipers! Since the early 1990s, Chrysler has been using premium EPDM seals as well as phenolic pistons. So Chrysler no longer recommends rebuilding or replacing the calipers when the brake pads are changed -- unless, of course, an inspection reveals the caliper is leaking, worn or damaged. Chrysler also says the caliper slide pins are lubed for life and should last life of vehicle. But these too should be carefully inspected and relubricated if necessary when servicing the brakes.
As for brake hoses, there is no recommended replacement interval. Today's brake systems can develop up to 2,500 PSI or more, which requires top quality hose materials. The newer EPDM hoses not only withstand higher pressures but are also more resistant to moisture permeation, which means there's less water contamination of brake fluid over time. Therefore, Chrysler sees no need to replace brake hoses unless a hose is damaged.
With the exception of Viper, all current domestic Chrysler products use high temperature DOW 1000 DOT 3 brake fluid, which has a boiling temperature of 550 degrees F and surpasses the government's minimum performance requirements for DOT 3 brake fluid. Chrysler says their fluid can absorb up to 3 percent water and still meet the DOT 3 specifications for boiling temperature. Consequently, they see no reason to change the fluid for preventive maintenance.
In the past, European brake systems used vented master cylinder caps and lower quality hose and seal materials which allowed moisture to contaminate the fluid relatively quickly. Because of this, the brake fluid had to be changed periodically. But this is no longer true. Most European and domestic vehicles use sealed master cylinders with EPDM hoses and seals that resist moisture penetration. So fluid changes are a thing of the past, Chrysler says.
Fluid contamination by wear particles from seals and internal corrosion can sometimes cause problems, and particles as small as 5 microns in size ( a human hair is 100 microns in diameter) may be large enough to affect the operation of an ABS modulator. Even so, Chrysler makes no recommendation to replace the fluid.
Chrysler's position on rotor refinishing is to resurface or replace as needed based on a careful inspection of the rotors. Reasons for resurfacing would include grooving or variations in thickness (too much runout). Some vehicles are more sensitive to runout than others. The rotors on Jeep Grand Cherokee may cause vibration or pedal judder with only 18 microns of runout. Ram trucks, on the other hand, can handle up to 180 microns of runout with little or no feedback.
Another reason to resurface rotors is to remove the transfer layer of material from the rotor's surface. Friction material from the old pads becomes embedded in the metal and may interact with the new pads if not removed.
A rotor should be replaced if it is worn down to the discard thickness or cannot be resurfaced without exceeding the minimum "machine to" thickness.
OLDER CHRYSLER BRAKE BULLETINS
* TSB 05-03-97 -- Covers a brake pull problem on 1994-'97 Dodge Ram trucks. The fix is to inspect the brakes, check alignment and install a front wheel shim if needed to eliminate the pull.
* TSB 05-04-97 -- Deals with reduced rear brake lining life on 2500/3500 Ram trucks. The cause, in most cases, is operating the truck in a heavily loaded condition. This shifts more of the brake load to the rear brakes, which accelerated lining wear.
* TSB 05-02-98 -- Clicking noise from the brakes in 1998 Jeep Cherokee. The cure is to install new rubber backed pad insulators.
* "Creep Groan" Brake Noise on 1998 Jeep Cherokee. The brakes are noisy when the brakes are cold. The problem seems to be moisture sensitive and is worse in cool, damp conditions. The cure is to replace the linings.
FORD BRAKE NOTES
Ford says it has no official policy on resurfacing rotors. As long as no shake, vibration or brake pedal judder is present and the rotors are not badly grooved or rusted, there should be no need to resurface them. Ford says it hasn't seen any problems caused by film transfer from the friction material.
If rotors do need to be resurfaced, they encourage their dealers to use an on-car lathe rather than a bench lathe to minimize runout.
One item that seems to cause some confusion when it comes to resurfacing Ford rotors is Ford's use of a single "machine to" spec on their rotors. GM and Chrysler include both a "machine to" thickness and a discard thickness on their rotors.
The "machine to" spec on a Ford rotor is the minimum thickness to which the rotor can be turned and safely returned to service. The number includes enough margin of safety so that the rotor should last as long as the pads. Once the rotor is worn below the "machine to" thickness, it should not be resurfaced any more or returned to service when the pads are replaced. It must be replaced.
The problem is Ford does not put a "discard" thickness on their rotors so technicians sometimes think the "machine to" thickness is the discard thickness and replace the rotors unnecessarily.
Ford says the main issue with rotor thickness is not rotor warping, mechanical failure or heat management, but the physical operation of the brakes. The rotor must be thick enough to prevent the caliper pistons from popping out if the rotor and/or pads are worn. The "machine to" spec they use allows for this and leaves enough thickness to assure safe operation of the brakes.
When resurfacing a rotor, the minimum recommended depth of cut .002 in. The maximum is .008 in. If you take too deep a cut, it will leave a rough surface finish above 100 microns that may cause problems.
As for rotor runout, sensitivity depends on the vehicle application. On Ford 4x4 trucks, the limit is .001 in of runout.
If rotor runout is excessive, it will lead to uneven wear that causes the thickness of the rotor to vary. This, in turn, will cause a vibration or pedal shudder when braking. Problems such as rust between the rotor and hub, hub runout and uneven torquing of the lug nuts can all cause runout that leads to uneven wear and pedal vibration. Always clean the hub/rotor interface when servicing the rotors, and apply a thin coating of antiseize to the mating surface to inhibit corrosion.
Ford says aluminum alloy wheels are much stiffer than steel wheels, and are much more likely to cause a rotor runout problem if the lug nuts are not tightened evenly.