Master Cylinder Diaphragms- Question and Musings

[Lucien Harpress]

So on my eternal quest of "fixing a bike where finding spares is next to impossible", I've got to musing about brake systems of motorcycles. Specifically the gasket/diaphragm of the brake master cylinder.

Long story short, the diaphragm for the rear master cylinder on my KZ1300 is torn, and I'm wondering what my options to fix it are. The problem is that this part 100% doesn't exist anymore. The cheapest option is adapt a different rear master (which I don't want to do), replace the lid and diaphragm with a custom machined replacement from Germany (something like 65 Euros, plus shipping to the US), or somehow fix/make/adapt the gasket I have now.

As far as the musings go, I guess I'm wondering how critical the diaphragm function of eliminating all the air from the reservoir is- I could probably make a new flat gasket out of gasket material just to keep all the fluid inside the master cylinder, but I'd lose the function of the diaphragm.

My other option is to just jam the part with the hole in between the lid and the master itself, crank the lid down, and hope for the best. Any gurus have thoughts?

[CYBORG]

it was raining today, no surprises there, so in the spirit of "spring cleaning", I decided to re shuffle some of the stuff in my shop, and came across one of those Chinese master cylinders. I decided to install it on my 1200 for something to do. When I opened it up to fill with fluid, I noticed it did not have diaphragm . More of a gasket. Doesn't leak, and seems to be working fine. So maybe diaphragm isn't needed. At lest the Chinese don't think so.

[Lucien Harpress]

I'm hoping so. I'm fairly certain I can come up with a gasket/o-ring to seal the lid to the MC body, I just wasn't sure how necessary the movement of said gasket was, brake fluid being the mysterious and PITA substance it is.

I may as well try it. Worst case it doesn't work, I need to repaint the MC again, and I spring for the custom German option.

Ah, experimentation.

[Sidecar Bob]

As the brake pads wear the caliper's pistons stick out more so more of the fluid is in the caliper and less in the master cylinder. As the volume of fuel in the master cylinder's reservoir lowers something has to make up that volume so that a partial vacuum doesn't restrict further flow of the fluid out of the reservoir in much the same way that a fuel tank needs to be vented. The bellows design allows the diaphragm to move into the space vacated by the brake fluid while preventing contact between the brake fluid and the air and more importantly the moisture in the air (brake fluid is hygroscopic).

If the KZ rear master had a remote reservoir like the 'Wings the solution would be easy. Maybe you could find a front master that uses the same cover and use the diaphragm from it.

[Lucien Harpress]

Unfortunately, I've got a perfect storm of annoying details working against me.

First off, the KZ rear master and reservoir are integrated, so no luck there. Second, I'm learning that Kawasaki was not as good at reusing parts as Honda was on different models, so the rear master on the KZ1300 is unique to that model. In fact, even the later ZN1300 Voyagers based off the same engine used different master cylinders.

I've considered grabbing a larger square gasket for a different MC with an integrated diaphragm and trimming it to fit, but getting dimensions on any of them without physically seeing them is proving difficult.

The joys of rare, one-of-a-kind bike ownership...

[Track T 2411]

How badly is your existing diaphragm damaged? I wonder if the glue recommended to patch 1200's carb diaphrams might work... Just a thought.

[Lucien Harpress]

The tear actually isn't that bad- it looks like a corner got pinched in the lid and it creased it enough to split it, about a half inch long. Searching for adhesive/silicone/rtv/whatever that can hold up to brake fluid is proving challenging.

I actually just ordered a GM/Chevy gasket I may be able use with only a small amount of modification. I'll know in a couple days.

[CYBORG]

As the brake pads wear the caliper's pistons stick out more so more of the fluid is in the caliper and less in the master cylinder. As the volume of fuel in the master cylinder's reservoir lowers something has to make up that volume so that a partial vacuum doesn't restrict further flow of the fluid out of the reservoir in much the same way that a fuel tank needs to be vented. The bellows design allows the diaphragm to move into the space vacated by the brake fluid while preventing contact between the brake fluid and the air and more importantly the moisture in the air (brake fluid is hygroscopic).

If the KZ rear master had a remote reservoir like the 'Wings the solution would be easy. Maybe you could find a front master that uses the same cover and use the diaphragm from it.

I understand the concept, and it makes perfect sense. However, these master cyl. don't have a rubber diaphragm, and they seem to work. Not sure how they do it, and should they stop doing it, I'll look into it. But for now I have enough other things to do, things to that I will live with the mystery

[Sidecar Bob]

The question is will it still work when the pads get worn and most of the fluid is in the calipers?

It is absolutely amazing how many people don't understand that the master cylinder's fluid level marks aret there to tell you when to replace the pads, not to tell you when to top it up. Maybe the people that put in a gasket instead of a diaphragm don't understand that either...

[CYBORG]

The question is will it still work when the pads get worn and most of the fluid is in the calipers?

It is absolutely amazing how many people don't understand that the master cylinder's fluid level marks aret there to tell you when to replace the pads, not to tell you when to top it up. Maybe the people that put in a gasket instead of a diaphragm don't understand that either...

I agree with every thing you are saying. And I am watching it very close,...and with great interest, for several reasons. I would never spend any of my money on one, and I have several Honda ones in stock as it were, but I'm very curious as to how functional they really are. I was using one of Randakks master cyls, 5/8 bore, and was happy with it but wanted a different look. I'm saving it for a bike I'm building that it will look better on. It looked a little out of place on my 1200. The Chinese one has no markings as to the bore size, but it does look better. I know, form over function. I've always been more of a function over form guy. Time will tell

[cafton10]

I see this is an older post but perhaps the information will be helpful to someone else.

What about the glue used for o-rings? I rebuilt the final drive on my '75 Goldwing a few years ago and could to find an o-ring. There is a company in Grand Rapids (Kent Rubber) that sold me material and glue. Cut to size, glue back together and good to go. As long as it's not under pressure this works perfectly. Seems like the same thing would work to repair the diaphragm. That may be just enough information to be encouraging but not helpful.

[Lucien Harpress]

I'm curious if anyone has come up with a sealer than can hold up to brake fluid, or at least DOT 3. My small amount of internet research has come up with not much, but I'm not ruling out that anything exists.

As it stands I attempted to cut down a different master cylinder gasket and failed miserably, so for the time being I popped my old diaphragm back in with the cut portion sandwiched back in between the MC and the lid like it was before. I need to see if there's any leakage after a couple of rides, but I've essentially turned it into a problem I can pay for later, IF I have to.

[5speed]

not sure if you have or not..but in case you haven't..give Andy a call. He may have the part or run into this situation before and has a solution.

http://vintagekawasaki.com/webshop/