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?

  • As it stands, this question is better suited for engineering.SE
    – Zaid
    Commented Sep 3, 2015 at 13:35
  • 3
    @Zaid - It would need substantial editing to be constructive on Engineering. From my reading, it appears that the OP is more interested in the mechanical aspects as opposed to the engineering theory or principles involved.
    – user2776
    Commented Sep 3, 2015 at 14:57
  • 2
    @Zaid This could definitely help someone understand the brake system better which will help them in diagnosing repairs. This is good knowledge even if the part isn't "serviceable". I like the premise of this question. Commented Sep 3, 2015 at 17:02
  • Although it doesn't seem to match common sense, in reality, vacuum does not pull anything. Air is constantly PUSHING on everything, and vacuum just gives the air a direction to push in.
    – barbecue
    Commented Sep 5, 2015 at 2:12

3 Answers 3


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


They is designed for the entire life of the vehicle. No maintenance ;except fluid check is commonly ever needed. Avoid disassembly unless you are convinced of failure.


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.

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