Clutch master cylinder reservoir

When changing the clutch fluid, is the rubber diaphragm that sits under the reservoir cover supposed to be fully protruding into the reservoir. This would mean that there would only be room for a very small amount of fluid.

The diaphragm in the brake reservoir folds up nice and small under the cover allowing the brake fluid to be filled to the top mark.

Just wondering Thanks,

I believe folded is the normal installed configuration.

The whole point of the diaphragm is to prevent air, laden with moisture, into the reservoir. Brake fluid is hygroscopic so it will absorb water reducing its boiling point and causing corrosion in the system. By using a diaphragm as the brake pads wear and the pistons get pushed out the level of fluid in the reservoir will drop. It has to be replaced with something so the cap of the reservoir is vented but the air is separated from the fluid by the diaphragm which will expand down into the reservoir as the fluid level drops.

The diaphragm should be installed 'Collapsed' with as little air beneath it as possible. This way there is little air in the system but the diaphragm can drop with the fluid level.

Thanks lads, that is what I was thinking. The problem is trying to get the clutch diaphragm to fold, The brake one folds up nice and small but the clutch one is a bit of a bitch, maybe because of low level fluid it got a bit deformed. I had neglected it and the fluid was shitty and low, the brake one was fine.

I got it back together but it was a bit of a struggle to try and get it to stay folded so that the fluid wasn’t pushed out all over the place, I probably need a new one for next time.

For some reason CARC bike's are murder on clutch fluid. I have no idea why!

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Pete Roper wrote:

The whole point of the diaphragm is to prevent air, laden with moisture, into the reservoir. Brake fluid is hygroscopic so it will absorb water reducing its boiling point and causing corrosion in the system. By using a diaphragm as the brake pads wear and the pistons get pushed out the level of fluid in the reservoir will drop. It has to be replaced with something so the cap of the reservoir is vented but the air is separated from the fluid by the diaphragm which will expand down into the reservoir as the fluid level drops.

The diaphragm should be installed 'Collapsed' with as little air beneath it as possible. This way there is little air in the system but the diaphragm can drop with the fluid level.

I've diligently bled the brakes & clutch on various bikes over the years but never realised this subtly.

Thanks

Looking at the exploded view in the parts drawing, it shows the brake reservoir diaphragm folded but the clutch reservoir diaphragm fully formed.

Maybe it is because there is a smaller chance of a drop in the fluid level in the clutch reservoir because it doesn’t have to compensate for the brake pad wear.

On my 2008 the diaphragm over the fluid in the clutch master cylinder will not collapse.

I know that it does in modern designs, but in this case I have tried to fold it in but there are no signs that it is meant to or ever has done.