VW brake proportioning valve regulates the amount of brake pressure sent to the rear wheels. It also compensates for heavy loads and panic stops to reduce the likelihood of the rear wheels locking up. If your rear brakes seem to be wearing out too quickly or lock up during heavy braking, you may need to replace this valve. This is the valve that should be used if you are converting from rear drums to rear discs.
When you change the brake proportioning valve, you will need to bleed the air and old fluid out of the brake system. Make sure you have a new bottle of brake fluid ready when you begin this job. If you don’t have a Bentley manual for your Volkswagen yet, add one to your order, so you know you are fixing your VW the proper way. A popular edition is shown below.