I believe it's been done before.
Here is one from 2015.
https://www.k100-forum.com/t10024-wanted-k100-rs-16v-front-brake-master-cylinder

I once replaced the front reservoir sight glass on a 4VRS with clear epoxy.  Worked surprisingly well and since I used clear epoxy the brake fluid level was visible.

1 - Remove from bike.
2 - Place tape over hole.
3 - Lay on side.
4 - Fill hole from inside with clear epoxy.

Hi, parky. Please click your cursor on the Profile tab in the header of this page then click on the Signature tab that will appear. After that, enter the year of manufacture and the model of your moto then save it. The year and model of your moto will appear below each of your posts, helping readers understand what you are riding without their having to backtrack to find the information. Parts might vary year to year and model to model.

Thanks for the ideas team. Ideally someone might know of a drop in aftermarket glass that fits the girl well and will provide years of maintenance free service. I shall keep hunting. Thanks again.

Well mate all I can suggest is a trip to the bike wreckers and sus out all the sight glass master cylinders on Jap Bikes for one that's close

Indeed, that's a good suggestion. I'm quite surprised that it has not been done and "threaded" in a forum. From what I can gather, there is a wee lip or step that makes this sight glass unique to the more ubiquitous (Jap) models. Motorbins has an exchange offer on such assembles where they recondition second hand units. My question is where do they get the glass to do this? So frustrating.

I asked the Google Gods, by incanting 'Brembo brake master cylinder lens' into the hallowed search engine vessel, and got dozens of various sized sight glasses, some with o-ring and clip, including for Brembo.

The sight glass, lens or window on my 20mm spare R11/K11 MC measures 20mm. The hunt is on.

Cheers me old china, I'll have another hunt. I'd love to see what it actually looks like - the profile et al. Once the piece is out, it's not going back in, and this this my daily ride. Thanks again for your troubles, much appreciated.

You really should try the epoxy thing I mentioned above.  It DOES work and is not irreversible in the event that you find a suitable sight glass in the future.

Alright, alright Duck, I will give it a whirl. And the beauty is that I be able to have a good look at the form factor of the old window. Thanks.

Sight glass on my '91 K100RS 16V is leaking too. Magura 20mm m/cyl. Window's not damaged so assumed to be corrosion around the edge, which would be consistent with the general state of neglect.

Has anyone been able to source a replacement window? Seems to be a larger diameter than more recent bikes.

In the absence of a new part, my thoughts are to press out the sight glass from the inside, clean up the housing, replace the o-ring and put it back together. Anyone done that?


Failing that, are any of the master cylinders on later BMWs, e.g. R1150, K1200, compatible with the K100 switch gear? Has anyone managed to fit a modern radial m/cyl, GSXR say, without changing the switch gear?

The first time I recall this problem was about 6 years ago and no sight glass was available then and AFAIK that is still the case. A few years ago I ended up making a small aluminium replacement and pressing it in but then there was no sight glass but it didn't leak.
If you find a better solution please share it.
I know that Rossco had the same problem and used some black sealant of some sort, a search may pay off for that. Not all the silicone sealants are brake fluid proof but he found one that worked.

RicK G wrote:I know that Rossco had the same problem and used some black sealant of some sort, a search may pay off for that. Not all the silicone sealants are brake fluid proof but he found one that worked.

Rossco seemed to sum up his repair in four posts.
Post 1.
Post 2.
Post 3.
Post 4.

Following on from #12, above...

I took the master cylinder off the bike, drained the fluid, put it in a bucket of water and blew some air into the outlet union. Sure enough, a steady stream of bubbles from the top left of the sight glass.

Popped the sight glass out with a bit more air. Although under water, the sight glass came out rather violently and sustained some damage in the process. Might have been cracked before but definitely cracked now. On reflection it might have been better to 'jack' it out with a non-compressible fluid (water). The damage is to the outer flange:



As anticipated, the master cylinder body was quite corroded and the seat for the sight glass o-ring was noticeably pitted. The body was stripped and left to soak in Coke (mild phosphoric acid) for a few days. Then cleaned using Scotchbrite and a shaped aluminium 'scraper'. The objective being to flatten the pitting as much as possible, removing all oxide but no metal. Even so, the o-ring groove was still pitted and not the finish you want for an effective seal.

Unfortunately, replacement sight glasses do not seem to be available and ones for oriental bikes don't fit. So it was repaired with cyanoacrylate adhesive (super glue). Don't know whether cyanoacrylate is resistant to brake fluid but it shouldn't matter since the repair was to the outer flange of the 'glass with serves only to retain the o-ring which, if it's sealing properly, should not allow brake fluid to reach the glued area. The repair is less visible in reality than it appears in these pictures.



The o-ring was well greased using the Magura supplied lube in the master cylinder repair kit and the sight glass pressed back in with a new 2x16mm o-ring (EPDM, note). The sight glass is retained by the o-ring which sits in grooves in both master cylinder and sight glass. The master cylinder was placed in hot water for a few hours to soften the o-ring in the hope that it would conform to the pitted surface. No air bubbles evident when lightly pressurised.



It is possible that the assembly grease is providing the seal, only time will tell, but three weeks later and so far so good.


While it was apart, I cleaned the master cylinder up and sprayed it with black EHT; while it looks good on an exhaust, it's not the best match for the brake original. It will do for now.

Would it be possible to turn a replacement glass up from perspex rod? Happy to have a crack at this, I must have a spare master cylinder somewhere that I can knock the window out of to copy.

BTW you're in Wrecsam - you're on lockdown and not allowed on forums!

Tool marks in the perspex will be hard to remove and won't let you see the level.

This may be a case where casting some acrylic in a mold might be the best solution. I've seen articles describing the molding of taillight and turn signal lenses. Clarity might be enough to make a functional window.

Make the pattern on a lathe. Or maybe 3D printing.

chris846 wrote:Would it be possible to turn a replacement glass up from perspex rod? Happy to have a crack at this, I must have a spare master cylinder somewhere that I can knock the window out of to copy.

BTW you're in Wrecsam - you're on lockdown and not allowed on forums!

It's spelt Wrexham.

I did look up brake fluid resistant materials and acrylic seems to be suitable. Only direct reference I found was a rather expensive engineering grade acrylic from Germany.

Tool marks can be removed by polishing. I made a sight glass for my radiator overflow tank using a drill press with a cross slide vice and wood routing tools. Marks were  buffed out using cotton mops and plastic polishing compound.
Regards Martin.