Hi dj95401. What I am referring to is not the same as the proportioning valves or pressure reducing valves you often see. It is a check valve, in fact, to be precise, it is a double or two-way check valve, although VW refer to it as a 'Residual Pressure Valve'. Its' purpose is to maintain a residual pressure in the brake circuit when the pedal is released in order to avoid excessive pedal travel when the brakes are re-applied. Here is a diagram:

As you can see, if the main valve was seized the pressure in the brake circuit would not be relieved.