VW brake check valve. This original equipment, direct fit check valve sits on a vacuum line leading to the brake booster servo. It allows vacuum pressure in the brake booster servo, and doesn’t let any out. If your brake pedal feels stiffer than normal, you will want to start investigating the check valve, and vacuum lines leading to the power brake booster.
If your VW brake check valve does not solve any of your braking issues, the vacuum leak is coming from another source. It is common for the power brake servo, shown below, to fail. Also your master cylinder could have failed. You can’t repair the master cylinder so it must be replaced. A popular master cylinder for the Golf and Jetta is shown below.