The composition of materials that the Porsche disc brake pads are made from is designed to disintegrate when friction is made between them and the brake rotors on your vehicle. It is not that the disc brake pads are made of a soft material; they are just softer than the metal side of the rotors they come in contact with. It is this principal of physics that the softer components will wear while the harder one will remain intact that permits the braking process on your Porsche to exist with no permanent damage being inflected on the components involved. As the Porsche brake disc pads wear, the piston in the caliper has to compensate for this loss of material by moving outward. This decreases the volume of brake fluid in the master cylinder on your Porsche. When the fluid level in the master cylinder appears to be low by the owner of the vehicle, no additional fluid should be added. This is an indication that the wear on the disc brake pads is extensive and they should be inspected. The brake fluid has moved to the caliper because the piston has moved out. The reservoir's size is made to allow for this without the need of any additional fluid to be added before a brake job can be done on the vehicle. If brake fluid is added, then when the piston is forced back into the caliper at the next brake job when the disc brake pads are replaced, the reservoir will over flow and spill brake fluid into the engine compartment of the Porsche. If the disc brake pads are not replaced at this time, then they will continue to wear until the padding material is depleted and the metal backing plate makes contact with the rotor damaging both components.