VW rear brake proportioning valve. The brake proportioning valve regulates how much pressure goes to the rear brakes. This part is load sensitive to adjust your brakes according to how much weight is in the vehicle. If you are noticing your rear brakes lock up more, or your rear brakes wearing fast, you should investigate replacing your brake proportioning valve.
Installing a rear brake proportioning valve will require you to bleed your entire brake system. Since the rear proportioning valve effects how much pressure is applied to your rear brakes, there is a chance your rear brake hardware has been damaged in some way. Whether it be from to little pressure glazing your pads, or too much pressure wearing them out faster, be prepared to at least install a new brake drum and shoes on your Golf, Jetta, or Cabrio.