Originally Posted by MC770
The vacuum check valve is that something I cous see with just my eyes if it's working or not?
No, you could not tell by just looking. You would have to remove and test it.
Air should only pass through it in one direction. You can use a vacuum tester or your mouth to check it.
Try to find a vacuum diagram for your car in your Chilton's or Haynes manual or in a Google search and see if the system has any check valves in it that are related to the brake system.
You could also use a vacuum tester to check the brake booster diaphragm for leaks.