If the brake vacuum booster makes vacuum on one side and the other is with normal atmospheric pressure, why doesn't it get sucked by itself towards the vacuum side? If the vacuum isn't enough to pull it, how does the brake piston return back to its place after braking? Isn't this done by vacuum as well? I have watched videos where the brake piston was very hard to push by regular hand so how can vacuum only pull it?
There are valves involved that operate when pressure is applied or removed from the brake pedal. Have a look at these simplified diagrams - http://www.agcoauto.com/content/news/p2_articleid/129