formsAuthentication.Signout() not working ?? ANYONE KNOW THE ANSWER

[Guest]

I am using forms authentication in my apps.
Every page I added:
Me.Response.Expires = 0
Me.Response.Cache.SetCacheability
(HttpCacheability.NoCache)

In the logout page, I use formsAuthentication.signOut().
BUT, if I press back I still able to access the page and
even the link which is supposed to be secure.

I have seen a lot of UNANSWERED similar posts.
Is this a known problem? or there is a mistake in my
implementation.
Appreciate if anyone could help me.
Thanks