We do this with our galaxy.
You have to do some kind of debug Text object (that is on a 0 scale/0,0 parallax layer) then have it Text: Set Text: scrollBarBackground.X
That way, you can determine exactly which X-coordinate you need to tell it to stop at.
Then, you simply have an event like:
Compare two parameters:
scrollBarBackground.X < minX
-----scrollBarBackground: Set X to minX
and
Compare two parameters:
scrollBarBackground.X > maxX
-----scrollBarBackground: Set X to maxX
Make sure you Pin any subsidiary objects to the scollBarBackground.