In recent times some other user have also had this problem.
The sight glass can not be replaced easily. Ask at your local dealer. If no solution, you need a new (or used) master cilinder.
You can also search the forum for "Schauglas" or "Bremsflüssigkeitsbehälter".
Search for "Schauglas" ist´s easier.
http://forum.gs-500.de/search.php?searchid=678399