Watch out here! A master page is not a page that you redirect to. A master page is just there to create a basic layout for all your site pages. You have to redirect to one of the pages which uses the master page e.g. Home.aspx or Default.aspx:
Response.Redirect( "Default.aspx" );
Get more insights about master pages here:
http://www.asp.net/master-pages/tutorials[
^]
Happy coding,
Morgs