New value on server side - how to get javascript to see it?

So I have a master page. on this page there is a hidden field control.
This value changes when the user selects an item from a drop down list, which is within a usercontrol. This dropdown list is within an UpdatePanel.
Now on a certain page, when I press a button it is doing an AJAX POST grabbing the value from the hidden field.
Problem is, it does not see the updated value.
 
is there a way I can make it see the updated value?
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