Gasket in break fluid reservoar?
 

Hi!
In my newly bought Mazda 3, -07, I have a bellows-like rubber "gasket" tucked into the cap of the break fluid reservoar. The thing is that the rubber thing is way to big to fit in the cap.
Should there be anything like that in the cap? Should there be an ordenary flat rubber gasket?
Do you have anything at all that between the plastic cap and the reservoar itself?

I know Mazda has repaired the hydraul unit for the anti-spin, so I wonder if this bellows-like thing should be somewhere else?

:Wconfused

Thanks!