A few weeks ago, my car developed a grinding squeakiness. Too stubborn to drain my coveted savings at the mechanic shop, I spent one early Saturday morning Skyping with my automotively adept dad, who talked me through replacing my brake pads. Cars have lots of parts, so it is easy to be intimidated into ignorance. Don’t be!
Above is a drawing of disk brakes that might be found on a car: a rotor spins with the wheel as it turns, a caliper clamps down on the rotor to arrest its movement and stop the wheel. It does this by virtue of a piston that pushes on the brake pads. Brake fluid activates the piston to move when a pedal is pressed. (Most bicycles also have calipers that squeeze together against the rim of the wheel, except they are pulled tight with a cable rather than pushed with fluid.) As you can see below, I’ve made a video to explain how properly functioning disk brakes work.
In the side view (:45), you can see that with normal function, the expanding fluid causes both brake pads to touch the rotor evenly because the the piston moves outward while the rotor body moves inward. In a real car, the space between the brake pad and rotor is probably a fraction of a millimeter. The front brakes account for 75% of your breaking power.
On my car, both brake pads were worn evenly on the right wheel, whereas on the left wheel one brake pad was completely worn out while the other almost brand new. This was a signal that there was a problem. I learned this quickly when I couldn’t get the assembly to fit together again with new brake pads. Uh-oh! Dad was off Skype and at the swimming pool. If you are ever in this situation never leave your brake pads off your car! Unlike on a bicycle, this is a connected system, and losing pressure will cause all four brakes to not work. Also your piston will hit the rotor, which damages both these parts.
So what happened? Let’s look at the caliper parts more carefully. The caliper is not actually directly attached to the car. Two bolts attach it to the caliper bracket, which is attached to the car with two other bolts. Unscrewing one caliper bolt allows you to pivot it upward to perform maintenance (as you can see in the gallery below). This screw threads into the caliper guide pin, which slides in and out of the bracket. In the video you can see it in red, with the rubber housing expanding and contracting as the pin moves.
In my situation, one of the caliper guide pins was rusted in place within the caliper bracket. Both the uneven wear and difficult reassembly were symptoms of this. Since I could not remove the pin to replace it, I had to replace the whole bracket to fix the problem. Below is a photo of the new real bracket and pins.