I bought the wife a 2006 GS500F that she loves and is perfect for her. But yesterday she wanted to clean her sight glass "because it was dirty" and she said it dissolved when she touched it.
In fact, the outer clear cover broke and she pushed in the part with the circle clip. As we all know, manufacturers love to make us buy complete cylinders when this happens but $130 or more is crazy.
I came up with a lot of possibilities from using rubber washers and a machine screw with an acorn nut, to a grommet filled with a short piece of clear plastic, etc. There were a lot of posts online about gluing pennies, using epoxy and also a couple places selling new glass and telling people to use RTV Gasket Maker.
Well, I came up with a perfect fix that costs just $1.00. I know my wife would never really check her level anyway so I decided to forgo the lens. I bought a rubber stopper (remember those test tube stoppers in school chemistry?) from Home Depot that is 5/8" X 13/16". UPC 030699159888. It's tapered and the small end is just slightly larger than the opening. But if you press it in firmly while using a small flat-head screwdriver to work it in from the inside you can get it started. Then I simply used a small pair of needle nose pliers to work it in further, eventually leaving about 1/4" outside and about a 1/2" inside. The plug does slightly touch the reservoir cover but it seals perfectly when screwed all the way down. It takes up little room so there's plenty of fluid left when filled.
The end result is a totally sealed master cylinder and the plug will only come out with a lot of force. It causes no interference and won't harm the fluid or break down. Granted, you can no longer see level but we all know when it's low by the feel of the lever and actual checking requires removing 2 screws.
I think this works much better than pennies, glue, epoxy etc.
I'll post pics soon so you can all see.