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
The front chamber is called a constant pressure chamber and holds constant vacuum. The rear chamber is called a variable pressure chamber, and varies from same vacuum as front chamber when no brake pedal is applied to atmospheric pressure when high brake pedal force is applied. The return force is provided by a return spring which pushes the diaphragm, valve/rod and pedal back.