Brake Fluid Leaking - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 5 (permalink) Old 09-11-2008, 05:16 PM Thread Starter
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Brake Fluid Leaking

I changed by brake fluid the other day. One thing I noticed is that the diaphragm of the master cylinder (43028), as the parts manual calls it, which is the rubber piece that fits on the bottom of the master cylinder cover, was stuck in the out position. The rubber got bent somehow and it was stuck like that. Anyway, today I was out riding and I noticed that there were little specks all over the right side of my new leather jacket(!!!). I traced it to the little hole on the right side of the master cylinder. This can't be normal. Did I overfill the master cylinder and this is just its way of getting rid of the extra brake fluid? Or is it related to the strange diaphragm? Do I need to replace the diaphragm?

What do you guys think?
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post #2 of 5 (permalink) Old 09-11-2008, 05:29 PM
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They should not leak. Normaly you can just push the diaphram back in with your finger BTW..

But you need to find out where and why it is leaking and fix it. If they in fact sell replacement diaphrams you should get one if you think it is the cause of the leak.


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post #3 of 5 (permalink) Old 09-12-2008, 02:56 AM Thread Starter
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I pushed the diaphragm back up when i put the lid back on when I was done changing the fluid. It seems to be working fine. I was just wondering if that could have been a reason why it was leaking.

As far as the leaking goes, I think I might have just overfilled the master cylinder. I removed some of the brake fluid and will see how it works next time I get a chance to take it out for a spin.
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post #4 of 5 (permalink) Old 09-13-2008, 12:43 PM
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That "diaphragm" is supposed to expand/open like that when the fluid level drops to take up some of the empty space so that the brakes can still operate like they should, as opposed to squeezing the "air in that space" and the fluid.

(this is the best way I know to explain it, hopefully its somewhat understandable)

But the point is, it's supposed to expand as fluid goes down, and when you refill the fluid, you're supposed press the folds back together.

If you filled the MC without closing the diaphragm back together, then putting the cap back on would force the fluid up and out over the side of the MC, so you will have fluid all over at that point.

Make sure the fluid is full, the diaphragm is closed up and the cap is tight. Then take a rag, and even some Q-Tips and clean the MC all around the lid, the base-everything really well, and after a few uses, it will be pretty easy to tell where the leak is coming from.

Last edited by 750Doug; 09-13-2008 at 12:47 PM.
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post #5 of 5 (permalink) Old 09-28-2008, 09:27 PM
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You can buy replacement diaphragm for under 10 bucks at

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