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Brake Service Limits

When you inspect your brakes, any parts you find that are damaged or worn out obviously need to be replaced. A brake part is worn out when it can no longer perform its original design function safely. It may be broken, leaking, falling apart, on the verge of failing, etc.

A brake caliper, wheel cylinder, master cylinder or brake hose that is leaking brake fluid is an example of a worn out part that needs to be replaced. A master cylinder that is leaking internally and allows the brake pedal to slowly sink to the floor under pressure is a worn out part. So too is a brake pad or shoe that is worn down to bare metal. The Motorist Assurance Program (MAP) defines worn out parts as being at or below minimum service specifications. This assumes the OEM vehicle manufacturer or a government agency provides a minimum service specification for the part in question

. Minimum service specifications do exist for most brake linings -- which means you will have to refer to a service manual for thebrake caliper specifications. There are also minimum service specifications for disc brake rotors and brake drums (which are cast or stamped on the parts themselves). But no such specifications exist for many other brake system parts, so here are some "rules of thumb" based on industry accepted practices.

As a rule, brake linings must be replaced when:
* Wear sensors are making physical contact with the rotors (listen for a squealing noise when the brakes are applied).
* Linings are worn down to within 1/32 inch or less of the rivet heads. If the rivet heads are flush with the surface of the linings, they are overdue for replacement. Postponing the needed repairs will cause grooving of the rotors or drums, which in turn may require replacing these parts also!
* The thickness of the pads is at or below the minimum service specification defined by the OEM vehicle manufacturer or applicable government agency. Typically, this is 1/8 inch with bonded linings on passenger cars and light trucks. Measure the thickness of the lining material only, not the lining plus the backing plate.
* Linings are contaminated with brake fluid, oil or grease. This can cause grabbing, pulling and uneven braking.
* Linings are cracking, flaking, falling apart or loose. In states where wear limits are prescribed by law (California, for example), professional technicians can only replace brake linings if they are worn below specifications, damaged or contaminated by brake fluid, grease or oil. Such always do not apply to the vehicle owner for do-it-yourself repairs.

Brake rotors and drums must be replaced if they are worn too thin. Thin rotors and drums are dangerous because they increases the distance which the linings must move to make contact, which may exceed the piston travel design limits of the calipers or wheel cylinders (causing them to leak, jam or fall apart). Thin rotors and drums also have less mass and are able to absorb and dissipate less heat. Thinner metal is also weaker metal which increases the risk of cracking and component failure. The minimum thickness or discard specification is usually cast or stamped right on the rotor or drum itself. Rotors must be measured with a micrometer, and drums with a drum gauge. If you don't have or don't use these tools, you can't tell ifbrake caliper the part is at or beyond the service limit.

Even when you do use the proper measuring tools, the numbers can be confusing if you don't understand what they mean. A discard thickness on a rotor is just that. The rotor must be discarded and replaced with a new one if the measured thickness is at or less than the number on the part. But the number of some rotors is a minimum "machine to" specification, not a discard specification. And some manufacturers (GM and Chrysler) give you two sets of numbers: a minimum "machine to" specification and a discard thickness.

Ford, for example, only puts a minimum "machine to" specification on their rotors. If you don't know this and think the number is a discard specification, you're going to replace a lot of Ford rotors unnecessarily. A "machine to" specification is the minimum thickness to which a rotor can be safely resurfaced and reused. The minimum "machine to" specification allows enough residual thickness (.030 to .060 inch) to allow for normal wear until the next brake job, whenever that might be.

There are service limits for rotor finish, too. Most vehicle manufacturers have traditionally said a surface finish of 100 microinches or less is acceptable on a freshly turned rotor. But many now say the rotor finish should be 60 microinches or less, with 40 microinches being ideal.

The problem is most technicians have no way of measuring rotor surface finish. It requires using a "profilometer," an expensive electronic instrument that drags a diamond tipped stylus across the surface to measure its characteristics. The cheapest ones cost several hundred dollars and the more sophisticated ones can cost thousands. The only other alternative is to use a "surface comparison gauge" that has samples of surfaces with varying degrees

If a rotor looks like an old phonograph record after it has been turned, it's obviously much too rough. Turning a rotor too quickly will cause a rough finish. Dull tool bits in a brake lathe can also cause a poor surface finish. Rough rotors can be noisy rotors. They also increase the time it takes for new pads to seat and don't provide the best friction surface because only the sharp peaks on the surface make contact with the linings.

The ideal rotor finish is clean, smooth and flat. Rotors (and drums) should be washed after they have been turned to remove any residual debris. You can also sand the surface briefly with a ball brush or #150 grit aluminum oxide sandpaper as a final step to further improve the surface finish. But this shouldn't be necessary if the rotors are turned at the proper feed rate and rpm, and are cut with sharp tool bits.

The service limits for rotor runout vary with the application, but should ideally be .002 inch or less. Some manufacturers allow .003 to .005 inch or more depending on the vehicle. But on sensitive vehicles such as a late model Chevy Malibu, the service limit is only .0015 inches.

The only way to know if a rotor is within this limit or not is to measure runout with a dial indicator -- yet few technicians actually take the time to do this even when there is an obvious problem.

If runout is greater than specifications, the resulting wobble in the rotor may be felt as a pedal pulsation and may contribute to uneven rotor wear and even more pedal pulsation down the road.

Runout at the rotor can be caused by a stack up of manufacturing tolerances at the factory in the hub and/or rotor, but is often caused by rust and corrosion growing between the rotor and hub. Runout can also be caused by uneven torque of the lug nuts. That's why it is so important to inspect and clean the mounting area between he hub and rotor, and to use torque-limiting sockets or a torque wench when tightening lug nuts.

Runout can sometimes be reduced by measuring the rotor, then remounting it on the hub to see if a different index position has less runout. There are also tapered shims that can be installed between the rotor and hub to reduce runout. After marking the high spot on rotor, install the shim with the notch opposite the high spot. Then re measure to make sure runout has been reduced.

The best solution for a rotor runout problem, though, is to resurface the rotor while it is still mounted on the hub using an on-car brake lathe. These tools will cut a rotor true to the hub and produce much less runout than a typical brake lathe.

For a rotor to brake smoothly, there must be almost no variation in its thickness. Many drivers can feel thickness variations as a pedal pulsation if there's more than .001 inch difference in the rotor. Measuring thickness variation requires a micrometer and requires 12 to 18 measurements around the circumference of the rotor to find the minimum and maximum thickness. Even then, the numbers may be misleading if tip of the micrometer happened to end up on a small pit that has little or no effect on the pads.

The best advice here is to just assume the rotors have thickness variation (and possibly runout, too) if the driver complains of pedal pulsation or you feel any pulsation when test driving the vehicle. Pulsations can be temporarily eliminated by resurfacing the rotors, but they often come back if the rotor has developed hard spots. The best long term fix is to replace the rotors.

One final service limit worth mentioning is one for which you'll find almost no factory specifications: noise. Everybody wants quiet brakes, but you won't find any decibel or sound quality specifications in a service manual. The bottom line here is this: if the noise is objectionable, it needs to be fixed.

Noises may include squeals, squeaks or rattles. If the brake pads on your vehicle are too noisy, switch to another brand. As a rule, pads with a high ceramic content are typically quieter than semi-metallic pads and some original equipment non asbestos organic pads.

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