This boot is on the plunger end of the master cylinder. Keeps dirt and contaminants out of your master cylinder. 1950-1977 Master Cylinder Push Rod Boots afr often the culprits leading to failed master cylinders. Keep dirt and debris out of you brake master cylinder is the function of this boot.
Learn more from the following Mid America Motorworks Knowledge Library articles:
Air Cooled VW Master Cylinder Typical Removal and Installation Procedure
Air Cooled VW Master Cylinder Typical Bench Bleeding Procedure
Pacific Parts International PPI