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