What best to use to soften older m/c diaphrams ?
master cylinder rubber diaphrams
- Thread starter DeanR
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Slydynbye
Will ride for Pie
A new one
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Beat me to it.
OP
OP
This is for my old XS1100 front m/c.. 25 bucks for a new one. Not gonna happen at double the cost of one for the ST.
I was thinking Sylglide as well, but don't know if that's such a good idea around brake fluid.
I was thinking Sylglide as well, but don't know if that's such a good idea around brake fluid.
Diaphragm? Old and stiff? What will it cost you if it ruptures when you apply the brake? You could try softening an o ring, but a not a flat sheet of rubber (said diaphragm). Regardless, brakes on a mc are like the tires - your life depends on them. Might as well start collecting old tires, soaking them in cod liver oil (shark liver oil in a pinch) for a week or two and getting a few miles out of them. 
are you talking about part 11 in the diagram, Dean? If so, all that seems to do is expand as the brake fluid level drops to keep the fluid from sloshing around. I'd guess if you keep the fluid level topped off it won't make any difference at all. Just remember that you topped off the fluid the next time you go to change the brake pads, and remove the excess fluid before prying the caliper pistons back to install the pads. I'll admit I have no experience with changing that part, so I'm happy to hear someone else's explanation of how critical that part actually is.
OP
OP
Yes, that's the part Doug. I agree with your thoughts. In addition I figured it may help with any possible moisture absorption.
The piece I have is not in terrible shape, just 40 years old.
In terms of functionality, pretty sure the brakes would work just fine if it were not even there?
The piece I have is not in terrible shape, just 40 years old.
In terms of functionality, pretty sure the brakes would work just fine if it were not even there?
CYYJ
Michael
Uh, I don't think that "pretty sure" is a high enough level of assurance when it comes to safety systems such as brakes.
I could accept "pretty sure it would work" for something like a heated seat or an intercom system, but for brakes, my minimum standard is "absolutely certain" that they will work.
The primary purpose of that rubber diaphragm is to provide a barrier between the outside atmosphere and the brake fluid in the reservoir, this to prevent moisture from the atmosphere coming in contact with the brake fluid. As the fluid level in the reservoir rises and falls (due to brake application) or falls (due to brake pad wear), or rises & falls due to temperature variation, air will enter the space above the diaphragm. The diaphragm prevents ambient air from coming in contact with the brake fluid. It also prevents rain & condensation from coming in contact with the brake fluid (be aware that the cap on the brake fluid reservoir is vented to atmosphere).
I have often seen considerable condensation on the top surface of that rubber diaphragm when I have removed the cap to service the brakes. Hence I think that it is essential to have a diaphragm that is in good condition installed below the cap.
Michael
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The acute problems I have had with old diaphragms was that they seeped fluid around the edge of the reservoir cap or cover. If the reservoir is not sealed the fluid within has to be at greater risk of absorbing moisture.
OP
OP
Never said I would even consider going without the diaphram. I would surely not.
Beeflips
(it's me...Greg) Returning some videotapes
I had a bad seal on my CX500 year before last, fluid leaked and crystalized, and it made a mess. ate the paint from the master cylinder. If you can find just the seal through brake masters or something, I would try that https://brake-masters.com
John OoSTerhuis
Life Is Good!
A short soak in denatured alcohol.
I don't agree with this analysis, but I can't claim to be an expert on the system so I'm interested in hearing a discussion on the topic if anyone's interested.
In the picture below, item 10 is the part that seals the m/c reservoir from the outside world, and serves as a gasket between the m/c body and its cap. The diaphragm is item 11, which sits below item 10 and has nothing to do with sealing the system from the outside world.
As the fluid level falls due to brake pad wear, air doesn't enter the system, its a sealed system. What happens is the small amount of air that is trapped inside the m/c body when you put the cap back on presses against the diaphragm and holds it against the level of the brake fluid so it can't slosh from side to side. As the fluid level drops, a very small vacuum is created due to the displaced fluid, but the trapped air continues to exert pressure on the diaphragm so it tracks the level of the brake fluid. Its accordion shape allows it to deflect lower as the fluid level drops.
I believe its entire purpose is to prevent the brake fluid from sloshing around in the m/c body. If it weren't there to do that, and the fluid sloshed around enough, it could potentially allow the small amount of air trapped in the m/c body to enter the brake lines.
I do not believe that the cap on the m/c body is vented to the atmosphere in any way, I've never seen any signs of a vent mechanism. And, when the fluid level is full and you move the bars around, wouldn't the fluid be able to leak out of that vent mechanism?
comments invited.
OP
OP
Cool. Thought my thread was gonna just be a quick easy question. I've not really heard any discussion here on this before.
That's always what I do whenever I've had the m/c lids off.
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Part #10 is hard plastic and presses evenly down against the diaphragm to ensure the diaphragm seals against the top edge of the reservoir when the screws draw the metal #8 down. The diaphragm is the seal that keeps moisture at bay.
Ah yes, I wasn't completely sure exactly how parts 10 & 11 mated together, I was thinking the diaphragm sat just a little inboard of part #10. But you're right, the lip of the diaphragm (11) does extend over the edge of the reservoir and provides the seal.
regarding the comment earlier that the cap is vented to the atmosphere, I see there is a fine groove in each end of the metal cap, but the seal made by the diaphragm along the edge of the reservoir doesn't appear to have any gaps in it that would allow air inside the reservoir. So maybe I misunderstood that comment initially, was he saying that the reservoir is sealed from the atmosphere, but air is allowed between the metal cap and the diaphragm? If so, what's the reason for that arrangement? There's a thin vertical edge on the diaphragm that aligns with the thin groove on the cap, so its not clear to me if that allows air to get under the cap, or if that path is sealed.
I'm curious to understand this better, if anyone has more comments to add.
edit: I did a little searching on this and apparently it is desirable to allow atmospheric pressure to enter the area between the cap and diaphragm to equalize the pressure as the fluid level drops due to pad wear. I thought it just pulled a small vacuum and it was no big deal, but apparently its better to keep the pressure equalized, so I learned something today. I had assumed before that the reason the diaphragm got pulled downwards as the fluid level dropped was due to the small vacuum below the seal from the displaced fluid, but apparently its also due to the pressure equalizing above the seal through the cap.
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Andrew Shadow
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My understanding is that the area above the diaphragm is open to atmosphere to allow the diaphragm to move with the brake fluid without creating a vacuum. Vacuum on the fluid side of the diaphragm is not desirable as you don't want to be pulling the brake fluid away from the pistons at the end of the line, as unlikely as I think that is to happen in such a small volume system.
CYYJ
Michael
Yes.