[AS2][CS4] Scrolling masked movieclip issue

Hello,

I’m trying to make a movieclip (contentMain) scroll on the y-axis (using mouse wheel, not pan) with the following code:

mouseListener = new Object();
mouseListener.onMouseWheel = function (delta) {
contentMain._y += (delta*4);
}
Mouse.addListener(mouseListener);

The problem is this simple code makes a movieclip scroll vertically but out of the masked area.
I don’t know how to give the movieclip an upper/lower scroll limit.
Could anyone give me hand?
(the attached file is saved for CS3)