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  1. #1
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    Ever wondered how a hydraulic brake calliper works?

    Ever since I was a kid I've never been able to pass up the chance of taking something apart to see how it works. The only difference with age is I've gained the patience to put it back together again

    Anyway, one of my brake callipers needed attention the other day, so I thought I'd share what I found for all the other big kids out there...

    It's a very simple design really - only four moving parts (or five if you include the oil!)

    In the first picture you can see all the components - the two halves of the calliper body, the white ceramic pistons, square profile piston seals, bleed nipple, and bolt that holds one end of the calliper together. The other end of the calliper is held together by the banjo bolt, which is hollow and also delivers the oil from the hose.

    Ever wondered how a hydraulic brake calliper works?-img_6250-683x1024-.jpg

    The close-up shows the recess the piston seal sits in (A), the oil channels drilled into the calliper body (B&C), and the threaded hole at the top of the picture where the hose attaches via the banjo fitting (D).

    Ever wondered how a hydraulic brake calliper works?-img_6253-1024x683-3.jpg

    This is how it works... When the lever is pressed, oil is forced down the hose and into the calliper through the banjo fitting. It passes through the small oil channels drilled into the calliper, to a recess behind each piston. You can see the opening of the channel from the banjo bolt (B) on the right of the close-up picture above. The opening on the left (C) goes to the bleed nipple to allow air to be bled out of the calliper. The other half of the calliper body only has the one channel to carry oil from the banjo fitting. The increasing oil pressure behind the piston forces it out, and it presses the brake pad onto the rotor - slowing you down. All basic hydraulics so far, but here's the clever bit - the rubber seal is a tight fit round the piston, but a loose fit in its recess in the calliper body. So, as the piston extends it drags the rubber seal with it, distorting its shape. When the lever is released, the 'squashed' rubber seal wants to spring back into shape, pulling the piston back in a millimetre or so with it (this is also the process that makes the callipers self adjusting as the pads and rotor wear - the piston will always be forced out until it touches the rotor, but will only spring back a fixed amount). Meanwhile, as the pistons recede, the metal spring between the pads forces them apart and away from the rotor.

    Very clever, but deceptively simple really!

    Dissembling and re-assembly of the whole unit was quite easy, so if only Shimano would do spare seal & 'O' ring kits I reckon they could last for ever!
    Last edited by PerthMTB; 04-10-2013 at 07:58 PM.

  2. #2
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    Aloha Perth,

    Great job with the explanation. I've been doing mechanics for years and have overhauled my share of brakes on all kinds of vehicles from cars, trucks, motorcycles, bikes. Mechanical things are so fascinating. Well done Perth.

  3. #3
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    Very timely topic. What does the profile of the seal look like? Square or round?


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  4. #4
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    Quote Originally Posted by scar View Post
    Very timely topic. What does the profile of the seal look like? Square or round?


    ****
    Usually it's a type of quad ring.
    Lead by my Lefty............... right down the trail, no brakes.

  5. #5
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    I was always amazed on how little fluid is in the caliper.
    Lead by my Lefty............... right down the trail, no brakes.

  6. #6
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    Quote Originally Posted by scar View Post
    Very timely topic. What does the profile of the seal look like? Square or round?


    ****

    Quote Originally Posted by ziscwg View Post
    Usually it's a type of quad ring.
    Actually, in this case I believe its a simple square profile ring, rather than the 'X' profile of a quad ring - anyway certainly not round. My calliper is back together again, but take a look at this close-up I took of pistons and seals...

    Ever wondered how a hydraulic brake calliper works?-img_6255-800x533-.jpg
    Last edited by PerthMTB; 04-09-2013 at 06:56 PM.

  7. #7
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    Ever wondered how a hydraulic brake calliper works?

    Excellent artical. It's nice to know how these actually work . Thanks for sharing
    Bill

  8. #8
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    what did you use to seal the two halves back together?

  9. #9
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    Quote Originally Posted by irv_usc View Post
    what did you use to seal the two halves back together?
    Theres two common ways of sealing. The first is an O-ring thats installed between the two caliper halves at the fluid crossover. The second is just Mr. Gasket type stuff, Thats what came on my Deores. (or at least I think... as there was excess around the caliper halves)

  10. #10
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    Quote Originally Posted by irv_usc View Post
    what did you use to seal the two halves back together?
    Cenzobear has it in a nutshell, but maybe I can expand a little.

    But first let me issue a disclaimer that the following description only applies to two-piece callipers that have a banjo bolt type fitting. There are one piece callipers like the XTR and older XT, and other two-piece designs that don't have a banjo bolt like Deore, M446 etc. which will differ.

    Anyway, the two halves are just bolted together with a bolt at each end. One of those bolts is the hollow banjo bolt which serves three purposes - joining the two halves, attaching the hydraulic hose to the calliper, and allowing fluid to pass through it and between the two halves of the calliper. A small 'O' ring sits in a recess to seal around the point where the banjo bolt passes between the two halves of the calliper. No other sealing is required as this is the only point fluid crosses between the two sides. Quite a smart design really - minimal points of failure!

    Name:  banjo.jpg
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    And for completeness here's a picture of the banjo fitting. Not my photo this time, just one I found on the Web. But you can clearly see the slot in the thread at the bottom of the banjo bolt which allows it to tighten the two halves of the calliper together, whilst still allowing the oil to flow past the threads and into the calliper internals...

    Name:  Banjo fitting.jpg
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    Last edited by PerthMTB; 04-10-2013 at 09:47 PM.

  11. #11
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    Quote Originally Posted by ziscwg View Post
    I was always amazed on how little fluid is in the caliper.
    That's the beauty of hydraulic systems versus pneumatic. Fluids are incompressible so it takes very little fluid movement to translate into powerful linear movements. Virtually every bit of pressure applied at the master cylinder ends up as pressure at the pistons. Pressure is a wonderful thing!
    I live with fear and danger every day. And on the weekends she lets me go mountain biking.

  12. #12
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    Ever wondered how a hydraulic brake calliper works?

    Quote Originally Posted by KevinGT View Post
    That's the beauty of hydraulic systems versus pneumatic. Fluids are incompressible so it takes very little fluid movement to translate into powerful linear movements. Virtually every bit of pressure applied at the master cylinder ends up as pressure at the pistons. Pressure is a wonderful thing!
    This is also why brake hoses are so damn stiff and hard to cut! But I did always think it was pretty wild how little fluid movement translates into raw power. It's ridiculously efficient.


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  13. #13
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    Where are the pictures?

  14. #14
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    Quote Originally Posted by wasatchmtb View Post
    Where are the pictures?
    Just saw this thread and I too would love to see the photos. Moderator, can something be done to restore these?

  15. #15
    rebmem rbtm
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    Click on this link and then click on Images, it's the only way of seeing the photos.

    This forum now deletes photos that are uploaded as 'Attachments' after a certain period of time.

    I think the only way to get photos to stay on threads is to upload them to your personal gallery and then add them to your post by right clicking on the photo to 'Copy Image Location' and then pasting that address when using 'Insert Image'.

    Example address from a gallery: http://gallery.mtbr.com/data/mtbr/50..._by_livius.gif

    The address as a inserted as a image:


  16. #16
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    Hi Guys,

    Thanks for your interest in the thread. Didn't realise the mods delete photos after a while, guess they want to save space on the server?

    Anyway, here's the pics from the original thread in the order they appeared....

    Ever wondered how a hydraulic brake calliper works?-img_6250-683x1024-.jpg

    Ever wondered how a hydraulic brake calliper works?-img_6253-1024x683-3.jpg

    Ever wondered how a hydraulic brake calliper works?-img_6255-800x533-.jpg

    All except the banjo bolt as that wasn't one of mine anyway, but if you just do a google search you'll find plenty of images of banjo bolts to see what they look like.

    Let's see how long they last this time....




    Edit: Hmmm, that didn't seem to work - still no images

    2nd Edit: Well I even tried starting a new thread just for the pictures Ever Wondered How a Hydraulic Brake Calliper works - pictures but that didn't work either. Seems images just won't display in my threads at the moment, even though they show as being attached in the 'edit' screen. Can't think what else to try, sorry!

  17. #17
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    Ah, Ok - looks like this losing pictures thing is a site-wide issue, not just me ...

    What happen to my thread pics?

  18. #18
    rebmem rbtm
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    Upload the photos to http://gallery.mtbr.com/

    With a PC, right click on the photo you've uploaded and select 'Copy Image Location'
    With a Mac, Control click on the photo you've uploaded and select 'Copy Image Location'

    Paste the 'Image Location' into the window that comes up when you click on 'Insert Image':

    Don't upload the photo using 'Attachments':

    Photos will always show up when they are inserted as an image from the gallery.

  19. #19
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    I think this will do it

    Name:  caliper5.jpg
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    Name:  caliper11.jpg
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    Name:  caliper21.jpg
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    Name:  caliper31.jpg
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  20. #20
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    Thank you PerthMTB!

    Are the seals replaceable? Have you found similar seals?

    Could they be replaced with O rings?

    My rear is dripping oil between the caliper housing and the piston. I'm going to disassemble it today.

  21. #21
    rebmem rbtm
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    Quote Originally Posted by wasatchmtb View Post
    I think this will do it
    But how long will those photos be viewable, it looks like you've uploaded them as 'Attachments' which disappear after a while.

    Gallery: http://gallery.mtbr.com/data/mtbr/500/caliper5.jpg
    v
    Attachment: http://fcdn.mtbr.com/attachments/bra...s-caliper5.jpg
    Last edited by cobba; 09-15-2013 at 11:11 PM.

  22. #22
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    Thanks for the thread. Do you have one on "how hydraulic disc brakes work" in general?

  23. #23
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    Question. Is there an advantage to using banjo bolts? It seems not all shimano brake sets use them, usually as they go up in price. Are banjo bolt equipped brakes worth the extra $$$$?

  24. #24
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    The big advantage is being able to position the bolt in the optimal position in for the hose to clear the frame/forks, so you don't have to have the hose going through some stress increasing bends to enter the caliper body itself.
    I don't crash, I just have slightly uncontrolled dismounts!

  25. #25
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    Ah! the pictures are back - in triplicate

    Thanks mods!

    Are the seals replaceable? Have you found similar seals? Could they be replaced with O rings?
    It would be simple to replace the seals, but unfortunately Shimano don't sell seal kits like most other brake manufacturers I haven't looked into whether other brands seals or generic seals would fit, but if they're the same size and made of a material that doesn't react to mineral oil, there's no reason they shouldn't. Normal 'O' rings wouldn't work, it needs to be a square profile for the piston retraction mechanism to work properly (see above explanation of operation).

    Question. Is there an advantage to using banjo bolts? It seems not all shimano brake sets use them, usually as they go up in price. Are banjo bolt equipped brakes worth the extra $$$$?
    There's three advantages to banjo bolts that I can see:-
    1. As Kiwiplague mentioned, they allow rotation of the hose attachment joint to allow more flexibility on positioning
    2. Reduces the number of openings in the calliper, and therefore potential for leaks
    3. Less parts in total, therefore reduced size/complexity/weight
    Like everything on bikes, I guess its a tradeoff in terms of weight/cost/functionality that only you can decide is worth it. The gold standard is a one-piece caliper like the M985 - light, small, minimal points of failure - but expensive to manufacture so you pay for the privilege!

    Do you have one on "how hydraulic disc brakes work" in general?
    Fraid not - but the only other part of the equation is the lever assembly, and if I ever have the need to take one of those apart I promise to add it to this thread!

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