Diagnosing Uneven Brake Pad Wear (cont.)
What Can Cause Uneven Pad Wear?
The key to correcting the uneven pad wear is in accurately identifying the condition responsible for it. Now that we've seen how a caliper pushes and pulls the pads against the rotors, let's consider the different ways brake pads can wear unevenly.
Inner pad wear
Inner pad wear usually occurs when the piston can't retract properly. The piston may be binding in a scored cylinder. The piston seal may be distorted or just plain worn out. Both of these indicate a caliper teardown and rebuild.
The pad may be binding on a corroded caliper bracket. If this is the case, you may be able to clean the corrosion with emory cloth and brake cleaner. (Remember to keep brake cleaner away from rubber parts.) After the sliding surfaces have been cleaned, apply a dab of brake lubricant.
Outer pad wear
Outer pads will wear prematurely if the caliper bracket or (more likely) the caliper pins are corroded or if they're lubricated with the wrong lubricant. Again, they should be cleaned with emory cloth and lubricated with brake lubricant. Improperly installed pin boots are another possibility.
Both pads on one side thinner than pads on opposite side
This is generally caused by a hydraulic problem, although it may also be caused by a sticky piston. More often, the cause is a restriction in the brake hose on the opposite side. The side with the premature wear may also have a brake hose with internal damage that acts like a check valve, preventing the release of the brake fluid.
It's also possible that there's a hydraulic restriction higher up the line than the brake hose on the side with the pad wear. For example, a faulty ABS modulator may not allow the release (return) of pressure on that side.
One other possibility is air in the hydraulic line on the side opposite the pad wear.
All pads wearing even but prematurely
Generally, this is caused by two-footed driving or even the choice of pad materialcertain pad linings simply wear more quickly than others. But most reasons for premature pad wear are not related to the friction material. Properly functioning disc brakes result in even clamping pressures being applied to both the inboard and outboard pads on both calipers. In most systems, if the brake system is working properly, the rate of wear on each pad should be close to equal.
One thing to keep in mind: the master cylinder must allow the brake fluid to expand due to the heat generated at the wheels. If the master cylinder does not allow for this expansion, fluid will be trapped under low pressure at the wheels.
If we're talking about the pads on the rear only (ie., front pad wear is normal), then check for binding in parking brake cable.