Mouse up problem...

I've made buttons with a gui that trigger a $buttonNote variable. What I'm trying to do is to reset the varible to 'Z' when the left mouse button goes up. I need the timing of the button press with a trigger on the screen.

Here's my code for mouseUp, but it doesn't seem to work...

/////////////////////////////////////
// Mouse up fuction
/////////////////////////////////////
function sceneWindow2D::onMouseUp(%this, %mod, %worldPostion, %mouseClicks )
{
    $ButtonNote = "Z";
    echo("mouse up");
}

It works for first level but not the next... Should this code go in the main.cs or game.cs file?? (Right now I have it in the game.cs file.)