Sys.Application.add_init(AppInit);

function AppInit(sender) {
    var prm = Sys.WebForms.PageRequestManager.getInstance();
    prm.add_initializeRequest(InitializeRequest);
    prm.add_endRequest(EndRequest);
}

function InitializeRequest(sender, args) {
    $('.ajax_loader').show();
    $('.ajax_loader').fadeTo(0, 0);
    $('.ajax_loader').fadeTo('fast', 0.4);
}
function EndRequest(sender, args) {
    $('.ajax_loader').fadeTo('fast', 0, function() {
        $(this).hide();
    });
}