Diagnosing Uneven Brake Pad Wear

We all know that brake pads wear out and have to be replaced from time to time. And a while back, we saw how to replace the front and rear brake pads. Generally, the front pads will wear out long before the rear pads, since 75% of the braking effort is handled by the front. And, generally, all four of the front pads (two on each side) will show the same amount of wear.

But this past month we've had a few Club members report problems with uneven brake pad wear. In this case, simply replacing the pads won't fix anything—the new set will wear unevenly, too. So it's important to determine why the pads wore out unevenly first.

Anatomy of a Caliper

The illustration below shows all the parts of the Vigor's front caliper, and the order in which they're assembled. Notice that the outer brake pad has one shim, while the inner pad has two. The inner pad also has the wear indicator.

The piston moves the inner pad. Some brake calipers have two pistons, one for each pad. But the Vig uses a single piston in a caliper that "slides, or "floats."

Front Caliper (Exploded View)

Let's see how this works. When assembled, the "moving" parts fit together as shown in the schematic illustration below.

Brake Caliper (Schematic View)

You can see that the brake fluid comes in behind the piston and pushes it forward. This pushes the pad into contact with the rotor.

As brake fluid continues to come in, it will continue to fill the area behind the piston and "pull" the entire caliper inward, so that the outer pad contacts the rotor also. Once the two pads are pressed against the rotor, there's no more room for more fluid and you won't be able to push the brake pedal down any farther.

When you release the brake pedal, the piston seal (see exploded view above) comes into play. It's a square-cut seal made from a rubber formulation chosen for its resilience—that is, its ability to return to its original shape. Don't confuse this with the piston boot. The piston seal has two functions: (1) it prevents pressure leakage between the piston and the cylinder and (2) it helps pull the piston back into the cylinder. It acts like a return spring to retract the piston.

Piston and Caliper Movement

In the animation above, we can see the brake fluid push the piston (and inner pad) forward, toward the rotor. As fluid continues to be forced in under hydraulic pressure, it "pulls" the caliper inward until the outer pad makes contact with the rotor.

When the brake pedal is released, the fluid will return to the master cylinder, helped by the spring action of the piston seal.

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 material—certain 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.