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

var mdlCtl;

if (document.getElementById('ctl00_hfModalCtlID') != undefined) {
    mdlCtl = document.getElementById('ctl00_hfModalCtlID').value;
}else if(document.getElementById('ctl00_ctl00_hfModalCtlID') != undefined) {
    mdlCtl = document.getElementById('ctl00_ctl00_hfModalCtlID').value;
}

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

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