[black and blue]

Well, I saw a cheap & easy fix concering the sight glass on a front master cylinder at a bike shop a few weeks back. Tha mechanic super glued a penny to the outside of the master cylinder. I guess he applied tha glue to the outside edge around the opening then reached inside the master cylinder and put tha squeeze on the penny till the glue set up. Then he refilled the resevoir, closed it up & bled the brake. As far as I know if, it is still holding. Hope this helps
Later tator