Hi,
Base on your description, you want to prevent the whole page postback when you click on the menuitem to direct to a new content page?
As you said, your menu control is on the master page. In fact, you can not implement the page no-refresh when you re-load the content page.
Because the master page class derives from the UserControl class. When the application is executing, the master page just like another child control. So you can say the master page is not a true page.
And, when the page loads, you can notice the navigationURL of the Browser address bar is the content page's, but not the master page's.
So I would suggest you use iframe instead of the master page to implement no-postback when you click on the menuitem.
The more about how the master page works, you can read this link: http://www.odetocode.com/Articles/450.aspx
Hope it helps.
Please remember to mark the replies as answers if they help and unmark them if they provide no help.
Yours sincerely,
Amanda Wang
Microsoft Online Community Support