Shimano Brakes M575 - hole in reservoir cap?- Mtbr.com
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  1. #1
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    New question here. Shimano Brakes M575 - hole in reservoir cap?

    I recently suffered from a spongy back brake, so replaced the pads, and then went to top up the oil by taking off the reservoir cap (see picture), and pouring in more oil. However, when I replaced the cap, and tried the lever, oil seeped / poured out of the small hole on the cap (see circled hole in the picture - sorry for the poor quality photo!). I hadn't really noticed the hole before, but I cannot understand how the brakes ever worked, with a small hole in what I took to be a completely sealed system!? I am clearly misunderstanding something, so I would be very grateful if anyone could tell me how I can top up the oil in this brake, and NOT have it pouring out this little hole (which I presume must have a valid purpose).
    Thanks very much in advance!


    Shimano Brakes M575 - hole in reservoir cap?-dsc_0432.jpg

  2. #2
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    The hole is to allow the bladder to expand and contract. You did put the bladder back in?

  3. #3
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    Thanks for the reply. I'm not sure about a "bladder", but there is a rubber diaphragm which I replaced with a new one. The part numbered #3 in this document: http://www.sjscycles.com/Instruction...5_Tech_Doc.pdf

    and in this photo:

    Shimano Brakes M575 - hole in reservoir cap?-36865.jpg

    However, I can't see how the rubber diaphragm would expand to cover the hole from the inside?

    Thanks again....

  4. #4
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    Same part, different word used.

    That hole is there to all equalization of the diaphragm so it can keep air out of the reservoir.

    Sent from my SM-G950U using Tapatalk
    Life on a bike doesn't begin till the sun goes down.


  5. #5
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    Oh, OK, thanks! So maybe the oil is seeping out because I haven't bleed it and topped it up "properly" using a bleed kit. I will try that and see if it resolves the issue.

  6. #6
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    Quote Originally Posted by GraniteMann View Post
    Oh, OK, thanks! So maybe the oil is seeping out because I haven't bleed it and topped it up "properly" using a bleed kit. I will try that and see if it resolves the issue.
    Your old diaphragm was leaking, probably why you got spongy brakes. Fluid shouldnt get past it so it can come out that hole. Though you may have simply forced fluid past.

    And dont try to top off that way, use the bleed kit. Nothing but a mess to be made

    Sent from my SM-G950U using Tapatalk
    Life on a bike doesn't begin till the sun goes down.


  7. #7
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    OK - thanks. I now have a bleed kit on order :-)

  8. #8
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    Sounds like you maybe put the oil on top of the bladder.

    Imagine that reservoir as being like the cistern for your toilet. It's full of water and you flush the water goes out the pipe at the bottom.

    Your bike brakes are a bit like this. As you use your brake the oil goes in and out of the pipe at the bottom. As it does, the level of oil in the reservoir goes up and down.

    The problem is that, unlike the cistern on your toilet, your brake reservoir is on a bicycle. It gets thrown around and turned upside down. If it was open, the oil would spill out. If it is sealed tight shut, the oil level cannot go up and down. So here's how it works.

    Imagine your cistern but instead of being open at the top, there is a rubber diaphragm across the top of the water. It is fixed to the sides all the way around so that when the water goes up and down the rubber diaphragm moves with it but stops the water spilling out of the top.

    That's how the reservoir on Shimano brakes works. Under the top cap, that you took off, is the rubber diaphragm. The oil is under it. The top cap holds the diaphragm in place. The hole is to let the air escape as the diaphragm moves up with the oil.

    Does that makes sense?

  9. #9
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    Perfect sense! Thanks very much
    I hadn't put 2 and 2 together and realised that the oil remains below the diaphragm. It's obvious now you spell it out!
    Thanks again.

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