Reset UIScrollBar using verticalScrollPosition?

I have a website with a dynamic text box on the stage. The three nav buttons each load in a .TXT file to the text box on mouse click. I am using the UIScrollBar component for scrolling the text that is dynamically loaded.

My problem is, when I scroll down in the text box, then click one of the nav buttons, the scroll bar doesn’t reset to the top when the new text loads. I’ve done some research and it seems like I need to reset the verticalScrollPosition property to zero, but I can’t figure out how to implement this. Flash states that it is an undefined property when I’ve put this line of code in to my script:

myScrollBar.verticalScrollPosition = 0;

I’ve also tried this, with no apparent effect:

event.target.verticalScrollPosition = 0;

I would really appreciate some help with this. Thanks so much!!