Some vac pumps hold a vacuum with a reservoir , once that pressure is contained in its reserve the pump will shut off . If that pressure falls like during braking the unit comes back on , giving the pump a rest is the idea . A gentlemen out of England has a YouTube channel as a mechanical run down of bits and pieces he has experimented with using Volvo or Audi parts to reach an understanding of how the systems work , .brittanica restorations llc