All cars, be it a BMW or Volvo use the same layout for the brake system. A disc brake system of your car has many parts, but one which almost all of us know is the Brake Pad – which consists of a friction material with a metal backing, and the Brake Rotor – a large circular disc, located just inside your wheels, onto which the brake pads press when you push the brake pedal. The brake system on most car is a really an energy transfer system, taking forward momentum, and changing it into heat, from the friction of the brake pads against the brake rotors. By design, the brake pads float just above the brake rotors, with very little clearance. This is done intentionally, you don’t want to push your brake pedal far, and you want immediate response on braking, so the distance the pads have to travel has been minimized. Some cars, like Mercedes, will cycle the brakes as you are driving in wet weather, to clean off any water on the brake rotors
All of this is good and fine, except for another property of brake pads, metal. In the good old days brake pads were often organic, and usually employing some form of asbestos. These days we know that asbestos is nasty material, and a carcinogen, so it’s use has largely been banned. Instead, brake system manufacturers have been using small metal particles, imbedded in the brake pad, to improve brake efficiency. Brake pads with metal particles are called “semi-metallic” brake pads. Now you know why brake rotors also wear, and why it’s common these days to simply replace the rotors with a complete brake service, you have metal rubbing against metal every time you brake. The nature of semi-metallic brakes can also make them squeal at low speeds, some cars like Volvo, are very prone to this. An ironic fact is the higher the quality of the brake pad, the more likely it is to squeal.
Sometimes a compound can be applied to the friction material of the brake pads to help with squeal, but this is often a short-lived fix as the brake pads wear, so does the anti squeal compound itself. One tip we often offer is to a series of quick slow speed stops to clean the brake pads, often this will minimize the squealing, even if for a short time.