Checkbok checked value not affected in page load

Hi,
 1. When I run the attached app and later uncheck the check box to update page manually and later press F5. The check box not set to uncheck although I used this code.

C#]
if (Page.IsPostBack){
if (Session[STR_IsAutomaticRefreshMode] != null && (bool)Session[STR_IsAutomaticRefreshMode] == false) {
_timer.Enabled = false;
 _chkRefreshMode.Checked = false;
 UpdateTargets();
}}

2. How this problem can be solved ?.

  Thanks,