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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?


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    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!

  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
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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
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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
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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!

  26. #26
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    Thanks guys, appreciate the explanation, always wondered what was up with the banjo bolts.

  27. #27
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    Thanks for the article.
    Since Shimano does not sell internal parts, I would try an industrial supply store which specializes in O rings etc. The best thing is to disassemble the caliper, clean it and take it to the store. Unless Shimano is using non standard seals (unlikely b/c it will be more expensive for them) there is a chance of finding replacement seals. If a piston is damage then a new caliper will be needed. You can buy the cheapest model which uses the same piston and transfer it to an upper model. My 2 cents. Bye

  28. #28
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    Question

    Quote Originally Posted by PerthMTB View Post
    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
Views: 4634
Size:  42.8 KB

    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
Views: 6523
Size:  87.2 KB
    Hello
    Can you explain how you managed to get the ceramic pistons out. When I squeeze by brake leaver without the brake pads inserted, the pistons don´t come out enough for to pull them our

  29. #29
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    Quote Originally Posted by Grego007 View Post
    Hello
    Can you explain how you managed to get the ceramic pistons out. When I squeeze by brake leaver without the brake pads inserted, the pistons don´t come out enough for to pull them our
    Good question!
    I get them out as far as possible while the calliper is still attached to the brake lever, by removing pads and gently squeezing the lever repeatedly. One piston will always want to come out before the other so I hold this one in place so the other one comes out as well.
    Otherwise, if you've already detached the lever and hose, you'll need to put a blanking bolt in the banjo bolt hole (new callipers sold separately come with one installed in that hole) and attach a bleed hose and syringe to the bleed nipple and push brake fluid into the calliper with the syringe - mimicking the action of the brake lever.
    Once you've got them as far out as possible by either of these methods, split the two halves of the calliper, and you'll have enough exposed piston to grip with a wrench and gently ease it out the rest of the way by rotating and pulling out at the same time. However, do this very carefully as the ceramic piston is actually surprisingly delicate, and can crack quite easily if gripped too hard or twisted in the bore of the calliper.
    Likewise, be very gentle putting them back in, and make sure they are aligned perfectly parallel to the calliper wall before you push them in with a flat preferably plastic implement (I use a plastic tyre lever).
    Good luck!

  30. #30
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    Hello

    Ok, got it. Will try tomorrow. Thanks for the feedback
    By the way, have you ever managed to find compatible o-rings?
    Regards
    Andreas

  31. #31
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    Quote Originally Posted by Grego007 View Post
    By the way, have you ever managed to find compatible o-rings?
    Haven't looked. After cleaning and re-assembling mine it worked fine, so I'm assuming just a bit of grit. But I too would be interested to know if anyone has found suitable seals?

  32. #32
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    Can you please explain how to take apart the lever? Just broke mine ))=: I was using a hole punch and hammer to get the pin on the lever off. And, broke the reservoir where the lever attaches...

  33. #33
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    Quote Originally Posted by deadmanriding66 View Post
    Can you please explain how to take apart the lever? Just broke mine ))=: I was using a hole punch and hammer to get the pin on the lever off. And, broke the reservoir where the lever attaches...
    You don't mention the make or model, but assuming yours is one of the recent Shimano levers like XT/SLX/Deore then there is a small grub screw that holds the lever axle in place - part 7 on the diagram below. I agree it's not easy to spot as it's hidden in a recessed hole behind the lever and covered by a rubber plug! It undoes with a 2mm hex key then the lever bolt should push out easily.

    If you've broken the cast body of the lever unit, I think your only option now would be a whole replacement lever unit, but you can pick up an SLX online for about US$30 if you shop around a bit. Otherwise the diagram below also lists all the various part numbers, and the general rule with Shimano is that if there's a part number listed for it then you can order it from Shimano as a spare thru your LBS.

    Ever wondered how a hydraulic brake calliper works?-m675-2.jpg

    Original diagram can be found here: http://si.shimano.com/php/download.p...-M675-3394.pdf

  34. #34
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    Thanks for the quick reply!! Wish i would've asked this Q before i broke them... I have the xt m785 but can't find that screw??

    And, yes i did brake the cast body ))=: Thought the pin on the lever had to be forced out with a hole punch.. I feel like such a DICK.

    Just ordered the complete part for $45 That's the cheapest i found.

  35. #35
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    Man!! I'm a DIY person...But, for the life of me i can't find that screw cause where it saying it is, it's just a small hole... By the looks of it... the only way to remove the lever is by banging out the pin... Which is how i broke the darn thing...

  36. #36
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    At the bottom of that hole is a small black rubber plug covering the grub screw head. You need to fish the plug out with a sharp tool - I use a seal pick, but a paperclip or toothpick would do in a pinch. Then you can insert a hex key and undo the grub screw.

    Here's a close-up from the parts diagram for the M785 lever showing the screw and its cover, labelled #8 in this case:-

    Name:  M785.GIF
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  37. #37
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    OMG! OK, got it.. I poked at that spot before and it was so dirty, it felt hard and not the softness of the rubber plug... That's why i couldn't understand...

    You rule my friend!!

    Thank you so much for your time!! I owe you a tall cold ale.

  38. #38
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    You're welcome - only wish I'd got the info to you before you set to it with the hammer!

  39. #39
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    I think, these must match McMaster-Carr
    XMCarbonSpeed CS-057 29er custom build
    https://www.strava.com/athletes/2293724

  40. #40
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    Great explanation. Thanks for posting.

  41. #41
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    how does air get out from the caliper side that has no bleed port?

    what is the thickness of the piston? how much exposed part before it dislodges?

    is the seal just a normal square seal?
    Canfield Yelli Screamy

  42. #42
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    Very cool - Rep given!

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