Quick Take Up Master Cylinder
A quick take up master cylinder is used along with low drag brake calipers. Low drag brake systems are more fuel efficient than standard systems because the pads are moved further away from the rotor after application. This results in less drag an better fuel efficiency. If a standard master cylinder were installed on this system the brake pedal would fade to the floor. This is because the extra volume of brake fluid necessary to take up this extra space between the pads and the rotor would not be provided. This is why the quick take up master cylinder becomes necessary.
This master cylinder contains a quick take up valve and a large primary chamber to provide the extra volume of fluid to compensate for the extra distance. The fluid bypasses the seal and provides the volume of fluid necessary during initial brake application. This quickly takes up the extra space before the pads contact the rotors. This is why when the quick take up master cylinder is faulty the vehicle will have a low brake pedal. The master cylinder is not providing the volume needed to take up the space between the friction material and the brake rotor.