User Controls :: Prevent User To Not Go Back Visited Pages After Logout?

I have 2 master pages Default.aspx is from Site.Master and some more pages that are from Admin.Master, I have used the code that to prevent the user from going back to previous pages after logout.

 Here is my code

function preventBack()
{
window.history.forward();
}
setTimeout("preventBack()", 0);
window.onunload = function () { null };

The problem I am facing Admin.

Master page i.e I have Home.aspx, AboutUs.aspx,Admin.aspx,AddItem.aspx I was unable to navigate between those pages also. how to solve this. I have tried other methods also, but still facing same problem.