VW master cylinder. The master cylinder on your Volkswagen Rabbit, or Scirocco sends pressure through the brake lines to stop the vehicle. When these wear out you will typically notice a change in the way the brake pedal feels (spongy, sinking feeling, or effort required) that is not explained by the normal brake wear parts. This master cylinder is for German built cars with the ATE power brake servo with a 20.64mm piston.
Flush fresh brake fluid through your brake lines when you change the VW master cylinder. Never try to reuse old fluid, over time it draws moisture from the air and then can't handle the heat generated while stopping. Keep the Bentley manual handy when you are installing the master cylinder. It will have all the instructions, and measurements you need when working on your Rabbit or Jetta's master cylinder.