Pressure switch will allows the pump to prime, and also will trip the ABS light.
You need to get a Bentley manual if you wish to service your ABS system.
Keep in mind that many parts of your ABS system are very costly upwards of $1000 per part.
This pressure switch retails for $480 from VW.
I don't like to discuss servicing the ABS system with people unfamiliar with the system, because the chance of harm to oneself is very high.
Your ABS system can produce very high pressures up to 210 bar/ 3045 psi.
I suspect you don't have a Bentley manual.
Search on-line for ATE MkII ABS information, you'll get info related to other car makes, but the basic system is similar. If anything, it will help you understand how the system works.
Special service equipment is required to service the ABS system, the Bentley tells you what tools and equipment are needed (this system is obsolete).