The ability of the Volvo disc brake pads to create friction without damaging themselves or the brake rotors and discs is thanks to the composite material they are made from. These materials the brake disc pads are made from are a combination of organics, ceramics, or soft metallic shavings by most manufacturers. As friction is created by the contact between the Volvo disc brake pads and the rotors, the pads turn to dust. With each application of pressure, more of the padding material will gradually turn to this dust. In time, the disc brake pads will have worn down and be in need of replacing on your vehicle. There is a simple way for the Volvo owner to know if the time for the replacement of their disc brake pads is nearing. This is by observing the level of hydraulic fluid in the brake master cylinder. As the pads wear, the piston in the caliper mounted to that wheel assembly will have to compensate for the loss of material on the pads. This will require the piston to move out and use the hydraulic fluid in the reservoir of the master cylinder to fill in the gap. If the hydraulic fluid level in the master cylinder's reservoir appears to be low, do not add more fluid. Instead, check the remaining pad material of the pads on your Volvo. The reason for not filling up the reservoir is because the fluid that has moved the piston out in the brake caliper is still in the system. When the new disc brake pads are placed on your Volvo, the piston will be pushed back into the caliper raising the fluid level in the brake master cylinder. If hydraulic fluid is added to the reservoir, then it will overflow during the next brake job and a mess will have to be cleaned up.