Line up two MCS with a button using startDrag

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1

I am trying to line up two MCS with a button using startDrag, but when I click to drag, the two MCS are not lined up, and the two MCS do not stop in the final position of the button that uses startDrag.
Look the example: http://img91.imageshack.us/my.php?image=teste17ha.swf

AS


var rolamento_mc:MovieClip;
rolamento_mc.onPress = function () {
    this.startDrag (false, this._x, 70, this._x, 290);
    this.onMouseMove = function () {
        mask_mc._y = _root._ymouse;
        fundo_mc._y = _root._ymouse;
    };
};
rolamento_mc.onRelease = rolamento_mc.onReleaseOutside = function () {
    this.stopDrag ();
    delete this.onMouseMove;
};

Sorry about my English.