Preloading contents loaded by LoadVars Object

Hi guys,
I was trying to create a preloader that preloads content loaded by the LoadVars() Object. For some reason, the getBytesLoaded and getByetesTotal variables of the LoadVars object kept retuning a null value. Below’s how I do it:

var news_lv = new LoadVars();
var url = "news.aspx"

//////////////////////Preload Content///////////////////////////
var bytes_loaded = news_lv.getBytesLoaded();
var bytes_total = news_lv.getBytesTotal();
var getPercent = bytes_loaded/bytes_total;

this.loadBar._width = getPercent*100;

//loadText is the variable name of a textfield that 
//resides within the loadText_mov mc

loadtext_mov.loadText = Math.round(getPercent*100)+"% Loaded";

if ( Math.round(getPercent*100) >= 50 ) {
	my_color = new Color(loadBar);
if (bytes_loaded == bytes_total) {
news_lv.onLoad = function(success)
          if ( success )
           //some other codes

Any clue what’s wrong? Thanks in advance.