Brake fluid is notoriously destructive to things. It eats paint & I cant imagine silicone ever holding it. Im gonna say youll need a new reservoir to solve this IF you want to keep that window. If youre willing to lose the window, which I dont suggest, then Id say empty the res, clean the res around the window, silicone an Oring on it & clamp a piece of metal to that Oring. Man, talk about look like crap,, but it should Work. Id only comsider that answer IF I was suck in the middleof nowhere & had to do that to get home.

That's a nice find Babyhog. I rebuilt mine and was wondering if I'd wasted my time because the sight window looks questionable. I'm going to order one now before I need it.

A simple cheapskate thought ....

If your glass is foggy but is still holding the fluid in, simply take the top off once a year to check the fluid level and don't fix the glass ever.

Spending time and money to fix what isn't necessary -- isn't necessary.

=============

If it does leak, take the master cylinder off the bike, clean it well with brake parts cleaner and pour the inside of sight glass recess slam full of two part slow setting clear epoxy all the way out to the aluminum and let it set up well for 48 hours.  

Once again, you now have a fogged sight glass that doesn't leak (at a total cost maybe $2 in epoxy) so now you take the top off once year to check it.           Big deal.

=============

The sight glass function is useless anyway -- proper set up after adding new pads is to fill the sucker up to the top so you can get maximum pad "out" wear movement backed up with reserve fluid in the reservoir.   This fill point is up over the sight glass anyway.

And to check it, you generally take the top off anyway -- who is gonna trust the sight glass since it sees a completely different picture according to how the bike is on the side stand OR how the handle bars are tilted.



Stop the leak, fill it up, and take the lid off once a year to check the fluid level.

Yes, I take the top off to check level anyway, but still try to put things back into their 'original' condition if possible. I also thought maybe an aluminum plug might do the trick too. As long as it doesn't leak, I'm a happy thumper...

Thanks!

and opening the reservoir lets in humidity.

Its vented anyway...

much nicer approach..

Let us form us a wild-assed guess --  

Klotz must make them some super special red tinted zero wear  -- expanding --  brake fluid ....


             (all brake systems transfer fluid from master to slave cylinder as the pads move in due to simple wear)


Remember, the volume displaced by the forward moving puck down at the bottom is almost equal to half of the entire reservoir up top, so expect to see some fluid level change up top as your pads wear out.   This is also why you cheat and fill the durn thing up all the way when putting in new pads, that way you don't have to mess with it quite as much.


======================


Justin is correct, your master cylinder reservoir is always vented to the humid outside air and it does get moisture contaminated (cloudy looking) over about 2-3 years and should be changed periodically in any case.    

Since this is similar to the wear out life of a pair of front brake pads it tends to remind you to do the pump out / change out of the fluid at the same time you are installing a new set of pads.