Help! Leaky Master Cylinder

After a ton of work on my rebuild project, the rear master cylinder is weeping brake fluid and I am bummed.
The leak is between the aluminum housing and the plastic reservoir (please see arrow in picture below). I haven't seen one leak like this before - this is an eBay special so maybe it was apart here before?? I've spent a ton of money on rebuilding it and hate to give up now.

Has anyone seen this type of leak and is there a fix??

I haven't looked at mine in a while, but IIRC it's similar to the squarish-shaped front master cylinders that Kawi used.
And if that's the case, you should be able to replace the o-ring between the reservoir cup and body. Kawi doesn't actually offer that o-ring, but you should be able to find a place that carries an assortment of them that you can match one up from.

When I rebuilt the front m/c on my 650 about 5 years ago I replaced that o-ring, and it's held up perfectly.
After seperating the cup from the body (remove the 2 screws and small metal plate inside the cup) and cleaning them up, I took both pieces with me to the local Pep Boys Supercenter and found an o-ring that fit just right.

Hmmm... I've never seen one that leaked in that area before. Retro seems to have the answer for it though with the new o-ring. I agree, the rebuild kits for the rear M/C are crazy money. One thing I did to my rear M/C was wet sand it up to 1000 grit and went over it with "Autosol" and it turned out way better than I expected, It looks like a polished billet part VS. a polished cast part, you might want to try it if you get some time. Doug

I forgot about the hidden internal o-ring. I rebuilt my front MC a while ago and replaced that o-ring after separating the reservoir from the body. As long as the rear MC has an o-ring that's probably the fix.

Any tips on getting that plate out of the MC to expose the screws? It seems to have some tabs holding it on. Doesn't look easy to remove without breaking - and it's not offered as a part - that's scary!

Also, does the rear have an o-ring for sure?? It's from a 650 SR (14mm MC installed on my MKII for custom caliper set up).

Thanks!

79MKII wrote:

I forgot about the hidden internal o-ring. I rebuilt my front MC a while ago and replaced that o-ring after separating the reservoir from the body. As long as the rear MC has an o-ring that's probably the fix.

Any tips on getting that plate out of the MC to expose the screws? It seems to have some tabs holding it on. Doesn't look easy to remove without breaking - and it's not offered as a part - that's scary!

Also, does the rear have an o-ring for sure?? It's from a 650 SR (14mm MC installed on my MKII for custom caliper set up).

Thanks!

I just happen to have a '78 KZ650-D (SR) parts bike gathering dust in my garage!

I'll be working on my bikes this weekend, so I'll remove the rear m/c from the SR and disassemble it to see "what goes where and why" while I'm out there and post what I find (with pics too if possible).
If I trash it in doing so it's no biggie, it IS a parts bike afterall.

I got it apart! I had to pull up on the plastic part to pop it out and that exposed the metal plate and screws. The o-ring area was full of some kind of gritty stuff. I'm sure it was leaking past it. I took every single piece apart and cleaned it up really good. I hope I can find an o-ring to fit. It seems to be about 3mm thick and 50mm O.D.

The o-ring is totally flat where it met the plastic cup - do you think this is part of the design or just flattened with years of use?? It would look more like a "D" ring if you viewed a cross section of it.

79MKII wrote:

I got it apart! I had to pull up on the plastic part to pop it out and that exposed the metal plate and screws. The o-ring area was full of some kind of gritty stuff. I'm sure it was leaking past it. I took every single piece apart and cleaned it up really good. I hope I can find an o-ring to fit. It seems to be about 3mm thick and 50mm O.D.

Dang, and here I went and took all of these cool pics of how gunky the one on my 650 parts bike was!:pinch:


I'm glad you were able to figure it out and get it apart.
Once you did get it apart, I bet you wondered why you were struggling with it? I know I did with my bikes front m/c.:blush:

79MKII wrote:

The o-ring is totally flat where it met the plastic cup - do you think this is part of the design or just flattened with years of use?? It would look more like a "D" ring if you viewed a cross section of it.

Nope, it should be a totally symetrical "o-ring".
You'll want to clean the cup and body parts up really good (I'm sure you know this) before taking both with you to match up an o-ring. I used a little bit of carb cleaner and a Dremel tool armed with a nylon bristled brush to get mine spotless without fear of grinding down or gouging the casting.
For me, using a slightly thicker o-ring (just enough that I had to apply a bit of pressure to re-seat the cup fully into the body) coated with a light film of grease did the trick to seal it and keep it leak-free these last 5+ years.

Wow! That one looks pretty bad! Yea, once I figured out how to get the plate out from the inside it was a piece of cake. I was afraid of breaking off those little tabs.

Do you remember what material your o-ring was made from? I've read that many common material types are not compatible with brake fluid. Something else to keep me awake at night!:laugh:

I just bought some nylon bristle brushes for my dremel - guess it's time to use them;)

I cleaned the entire area around the o-ring, replaced the o-ring and the problem appears to be fixed. No more leaks! I did some research on o-ring material types and found that EPDM (ethylene propylene diene monomer) is recommended for automotive brake systems with glycol based fluid (DOT3 and 4). I had a really hard time finding the right size. I went to a bunch of auto parts stores, some industrial supply stores, and even a place specializing in seals - all with no luck. I was finally able to get the correct size, in the correct material through McMaster Carr. I had to buy 50 of them though, so if anybody needs one, let me know! For stopping the leak, it was money well spent!

I think I used some generic o-ring in the front master cylinder - I hope it holds up to the brake fluid.