Alpha mask animation

I am trying to animate the mask_mc from left to right using TimelineMax. I need a simple setup to set the position then move it across. Help is greatly appreciated.

var mask_mc = this.mask_mc;
var maskedContent = this.maskedContent;

//AlphaGradientMask
maskedContent.filters = [
    new createjs.AlphaMaskFilter(mask_mc.cacheCanvas)
];

maskedContent.updateCache();

// create timeline
var tl = new TimelineMax();

tl.to(mask_mc.getChildAt(0), 3,{x:400, onUpdate:updateMask}, 'start')

//AlphaMaskFilter needs to be updated on every tick if it has to tween.
function updateMask(){
    mask_mc.updateCache();
    exportRoot.maskedContent.filters = [
        new createjs.AlphaMaskFilter(mask_mc.cacheCanvas)
    ];

    exportRoot.maskedContent.updateCache();
    console.log('updating');
}