//  register for our events
Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(beginRequest);
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endRequest);    

var mdlCtl = document.getElementById('ctl00_hfModalCtlID').value;

function beginRequest(sender, args){
    $find(mdlCtl).show();
}

function endRequest(sender, args) {
    $find(mdlCtl).hide();
}