An ounce of prevention is worth a pound of cure.
This old axiom provides a concise summary of why brakes should be regularly inspected and cars should be brought in at the first sign of brake trouble. What could have been a $200 repair bill can easily grow to $600 or more with just a few more miles of driving.
“Metal to metal” is the term used to describe brake pads or shoes that have worn through all of their friction material, and the metal backing of the pad or shoe is now rubbing on the rotor or drum (also made of metal). When brakes go “metal to metal”, there is a noticeable low pitched grinding and vibration when braking, especially at lower speeds. Continuing to drive once brakes go metal to metal brakes will quickly increase your repair bill. The picture on the left is a set of brake pads that was ignored for so long that the metal backing was ground thin enough to be ejected from the caliper. The only thing left of the pad on the right is the anti-squeal shim.
The brake pad backing and the brake rotor are roughly the same hardness. When the friction material has worn out, the pad backing and the rotor make contact and both wear away very quickly. A rotor that might have been machinable shortly before the brakes went metal to metal, now may need to be replaced. Rotors will eventually wear to below the minimum thickness even if the pads do not go metal to metal. However, letting the pads wear beyond minimum thickness will hasten the need for rotor replacement.
The rotor on the right went metal to metal on the inner (right) side. Both sides of the rotor were the same thickness. Now the inner side has worn down to almost half the thickness of the outer. Car manufacturers publish minimum thickness specifications for their brake rotors. If the brake rotor falls below the minimum thickness specification, it must be replaced.
Rotors are not the only components that can be damaged by ignoring brakes in need of service; the brake calipers can also be destroyed by continuing to drive on bad brakes. Once a pad is ejected or worn too thin, the caliper piston can hyper-extend and come out of the bore, allowing brake fluid to leak out. When this happens, the calipers must be replaced. The calipers are not normally replaced during a brake job, so this is an extra expense that could have been avoided.