Front Master Cylinder Rebuild on Rear?

Wow... I admit I have never seen this before. And I have no clue how to fix that. I haven't seen any replacement parts for this. And how is the level window?
What I'd suggest is cleaning all up really good then taking a second look. Pause and think.

About the paint - not rubbing your nose in, just thinking about that plastic stopped me from any drastic heat treatment. I painted the master cylinders with black primer then gloss black meant for honestly I don't even remember now, but definitely engine related. I ignored any treatment suggestions except heating it up for the hell of it on an electric heater. Bottom line is they all look superb because I avoided spilling any brake fluid.

The rear master cylinder - whatever they came covered by factory, that black peels off from what I've seen. I left mine aluminum polished and I like them this way. For stock black look, I'd say pain them and be meticulous with handling the brake fluid.

As I haven't seen that m/c without the melted plastic piece, I suspect it's just a cover and there might be more 'stuff' going on in there. I'd be careful as you life might depend on this little piece.

Well that's pretty ugly Lucien. Technically, it shouldn't have happened at 200 F. Something's a little funky with your heating oven. All of the plastic products shouldn't melt until over 330 F. But it's done now, so how to recover?

The cavity that the plug sits in will not ever see pressure since it's cross drilled in to the reservoir so all you're looking for is a simple "plug". Nylon round bar is the most reasonably accessible. Torlon or Delrin are also available but typically are used as bearing material. All 3 would be suitable as far as compatible with the brake fluid.

A simple interference fit should probably suffice. There's no indication of a circlip groove so it must have been a pressed fit in to the casting. I would machine up a plug with a shoulder on it to limit the depth the plug sits in the bore so that the crossed drilled passages are still open to the reservoir. Probably a .006" - .010" interference fit if using Nylon would be my best guess. If using Torlon or Delrin, I would think maybe a .005" - .008" interference fit. That's my 2 cents.

I remember that you and Bruce (trikebldr) were tight. Haven't heard from Bruce in a loooong time. I would also ask him as I trust his expertise in the engineering field. If you do converse with him, tell him KB thinks of him often and hopes all is well in his life.

Cheers,
KB

Paint with regular rattle can Satin Black paint and let it dry, then use SILICON BRAKE FLUID, which is non corrosive.
The paint lasts forever then as its not eaten alive by horrible regular brake fluid.
Works great on your calipers too !

Pete F
UK

Admittedly I tend to overdo the paint on things, because I would like it to last as long as possible. This isn't even the first time I've done this- I used the same paint-and-bake method in the front MC, with no issues. Of course, they didn't use a plastic plug to drill the cross holes on the front MC either, so....

The sight glass, however(on the front), seems to be fine. Maybe the rubber seal has a higher melting point than the plastic. Either way, I'm not overly concerned with that.

As far as fixing the rear, my cheap idea was to find an EPDM rubber plug (which, far as I figure, is DOT-3 safe) of the correct size, then using some sheet ABS plastic I have lying around making a disc to clean up the outside.

The expensive option is to get in touch with a machinist I know and see if he can whip anything up.

Either way, it's been a..... "fun" learning experience.

Another approach - Check out these sites Lucien - Should find one that will press-in........ with a little epoxy for good measure !

coreplugs.co.uk/

www.hubbardspring.com/

Lucien - Clean the inside of that opening to bare metal and mic. it - if you would. I have a couple of other ideas regarding a plug.