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Brakes rub after used.

geomickbgeomickb Posts: 147
edited June 2013 in MTB workshop & tech
Hi,

When I have used my brakes on a section of downhill the levers become really stiff and the pads start to rub. I can push the pads apart and they are ok for a while. Any ideas why?

The brakes are the old Giant MPH ones (2006?)

Cheers,

Mick
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Posts

  • The RookieThe Rookie Posts: 27,747
    Sounds like the pistons are sticking, remove caliper from bike/forks, remove pads, use the lever to move the pistons out CAREFULLY, then clean around the exposed part of the piston using the same fuid as in the brakes, lean any crud from round the holes in the caliper the pistons are in, push back pistons, repeat.
  • levolonlevolon Posts: 78
    does this happen when brakes get hot?

    if yes then id think your system is overfilled so as fluid expands you get fluid lock
    bleed a bit of fluid off "one lever pull" and it should be ok again.
  • Chunkers1980Chunkers1980 Posts: 8,035
    That's bad advice, if you or anyone else has not touched then since they worked correctly.
  • cooldadcooldad Posts: 32,904
    Particularly as the pistons return when pushed.
    I don't do smileys.

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  • The RookieThe Rookie Posts: 27,747
    There is an air pocket (the dry side of the diaphragm) which could cause this issue, chances are just about nil though.
  • levolonlevolon Posts: 78
    edited May 2013
    cyd190468 wrote:
    lon 456 wrote:
    does this happen when brakes get hot?

    if yes then id think your system is overfilled so as fluid expands you get fluid lock
    bleed a bit of fluid off "one lever pull" and it should be ok again.
    This really only happens in car type open circuit systems if there isn't sufficient air space in the reservoir. MTB brakes are a sealed system (that's why you have to wind the levers out as the pads wear) so if there's too much fluid in the system the pads can't retract as there is nowhere for the fluid to go to.



    if his fluid is 6 yr old and hydrated so all the water content is in the caliper "water is heavier than brake fluid" it will cause this issue and gives the same issue as over filled so removing a little fluid at the bleed nipple will expel the worst of the hydrated fluid.
    hydrated fluid expands very quickly and cools quickly too, so pads move back again as fluid pressure drops.
    as op reported brake bind after downhill use it would suggest old fluid or over filled.
    30 years of motorcycle brake repair so have a bit of an idea on how brakes work and why you have a max level when filling.

    could be that op system has been filled with old pads fitted , and fluid should be done with pistons fully in so correct fluid level achieved ...
    you can have a read here about brake fluids if you like http://www.bikerumor.com/2013/04/11/tec ... c-updated/
  • geomickbgeomickb Posts: 147
    Fluid is old and the possibility of it getting hydrated was my thought.

    This behavior started happening practically overnight when no changes gave been made to the brakes for months.

    They work fine when cold so cannot be pistons sticking?
  • stubsstubs Posts: 5,258
    cyd190468 wrote:
    (that's why you have to wind the levers out as the pads wear) .

    :? What brakes do you have I have never had to wind anything out as the pads wear.
    Fig rolls: proof that god loves cyclists and that she wants us to do another lap
  • The RookieThe Rookie Posts: 27,747
    Giant MPH/Root have a variable volume M/C that you wind the adjuster knob on to compensate for wear.....

    http://singletrackworld.com/forum/topic ... mph-brakes
    Check out the knurled knob on the resevoir.
  • levolonlevolon Posts: 78
    edited May 2013
    brake fluid , reasons why it gets hydrated
    http://rdabrakes.com.au/page/TB04.aspx

    think the new x9's have a pad contact setting adjuster but i wont be buying them as a tadl dear for me..

    ill stick with my 596 deores as they work well with the rt discs
  • The RookieThe Rookie Posts: 27,747
    HELLO - It's in a SEALED system, if water can't get in, it can't get in the brake fluid (and if it could, brake fluid would be leaking out....)
  • levolonlevolon Posts: 78
    HELLO - It's in a SEALED system, if water can't get in, it can't get in the brake fluid (and if it could, brake fluid would be leaking out....)


    HELLO read the link !! your wrong , hard as you may find that :)
  • The RookieThe Rookie Posts: 27,747
    I read the link you twit, it's for open reservoir car systems (despite saying they are sealed they are not, turn a car upside down and the fluid leaks out)....the open resrvoir allows moisture laden air in and out via the vent and that gets absorbed...

    So again, how does water magically enter a bikes sealed system?
  • levolonlevolon Posts: 78
    I read the link you twit, it's for open reservoir car systems (despite saying they are sealed they are not, turn a car upside down and the fluid leaks out)....the open resrvoir allows moisture laden air in and out via the vent and that gets absorbed...

    So again, how does water magically enter a bikes sealed system?



    you know fook all about brake fluid it would appear and your advice on brake maintenance is often dodgy "ie brake pads dont get contaminated " lol even though brake pads are porous !!! but you know better

    read this link then as it obviously needs to say mountain bike brake for you to understand!!
    http://www.reddogracing.co.uk/brakebleedfaq.php

    brake fluid is hygroscopic and absorbs moisture from the second you open the bottle or fill your sealed system..

    car bike and any other hydraulic system are all the same in principal and WHY you need to replace YOUR brake fluid periodically l
    weather its sealed or open it is the fluid that changes after time regardless

    so take you abuse and stick it where you talk from
  • The RookieThe Rookie Posts: 27,747
    I think after working in the car industry as a dveelopment engineer for 24 years I know a fair bit, it is a sealed system, tell me how any more moisture gets in than is there on day 1 and I may believe you.....it can only absorb moisture that gets to it, it can't create it from thin air (or brake fluid) to abosrb...forget engineering use common ruddy sense, a bottle of fluid can only abosrb the moisture in it, it then stays at that level until it is opened and more let in!

    So mr 57 posts, go away and come back when you can tell me how the water gets in.
  • OuijaOuija Posts: 1,386

    So again, how does water magically enter a bikes sealed system?

    Osmosis.

    Small water and air molecules can pass through larger molecular structures including plastics and rubber seals. Just requires time.
  • levolonlevolon Posts: 78
    Ouija wrote:

    So again, how does water magically enter a bikes sealed system?

    Osmosis.

    Small water and air molecules can pass through larger molecular structures including plastics and rubber seals. Just requires time.


    quite :wink: but he wants to make a c**t of himself now.. and doing a fine job of it too :lol:

    Beginner
    you would think a development engineer with 11.000 post would be able to spell DEVELOPMENT!!!
    all 11.000 post's on a forum means is you have 11.000 post's and this does not make you a GURU just a knob who dont know when he's beat...
    see we can all be condescending .

    im older than you Mr fat middle aged bloke with low blood pressure btw and i also have had 30 yrs working with brakes
    ill leave you to post away as you dont ride much obviously ..

    you have a nice day
  • The RookieThe Rookie Posts: 27,747
    Osmosis, Oh for heavens sake, what part of the system is a porous membrane, you do remember you need a porous membrake, viton for one is not porous......if you have 30 years working on brakes, I pity the poor people who have been paying you.
  • cooldadcooldad Posts: 32,904
    What happens if you use your middle finger to brake?
    I don't do smileys.

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  • stubsstubs Posts: 5,258
    cyd190468 wrote:
    !. Mountain bike brakes have been pretty much all fully sealed systems since about 2000ish. They are quite different to automotive or motorcycle brakes.
    2. While it may be possible for water to penetrate rubber seals osmotically it would literally take years so wouldn't be noticeable as a sudden change in brake function as reported by the OP.
    3. On my current bikes I have juicy 3s juicy 5s and m596s all of which have a lever position adjuster to compnsate for pad wear. (If you're a one finger braker you wind them out a bit when the lever starts to squash your middle finger) :)

    1. Wrong most MTB brake systems are open circuit apart from old Hope, Magura and Giant possibly some other old systems as well. MTB brakes are in fact principally the same as non ABS car and m/bike systems.

    2. True but thats not usually how water gets into a brake.

    3. Massively wrong, Dangerously wrong, all 3 brakes are open with a master cylinder reservoir. If your adjusting with the lever position adjuster to compensate for brake pad wear there is something badly wrong with the brakes. All 3 brakes you mention are self compensating get them serviced by someone who knows what they are talking about.

    Try doing some quick googling before you spout about something you dont know about, someone could take your censored advice and get hurt.
    Fig rolls: proof that god loves cyclists and that she wants us to do another lap
  • stubsstubs Posts: 5,258
    Here you go it took me all of 30 seconds to find this on Google. How hydraulic brakes work

    It tells you all about it and probably the Birds and the Bees as well
    Fig rolls: proof that god loves cyclists and that she wants us to do another lap
  • The RookieThe Rookie Posts: 27,747
    stubs wrote:
    1. Wrong most MTB brake systems are open circuit apart from old Hope, Magura and Giant possibly some other old systems as well. MTB brakes are in fact principally the same as non ABS car and m/bike systems. .
    Twaddle, show me the vent hole (to fluid) on a bike break (where fluid will leak out if you invert the bike).

    All the ones I have worked on use a diaphragm or volume adjuster on a sealed system, they may have an air vent above the diaphragm, but no air reaches the fluid.

    Car and motorbike systems have a vented cap, invert the car/bike and fluid leaks out.
  • stubsstubs Posts: 5,258
    Jesus wept do any of you guys know what your spouting about. Just because there is no vent hole it doesnt mean its a closed system. You all claim to be engineers with more knowledge and experience than God and probably work on repairing X wing fighters for all I know but you dont know the difference between a closed and open hydraulic system.

    I give up and bow to your 50 years experience of working on everything.
    Fig rolls: proof that god loves cyclists and that she wants us to do another lap
  • The RookieThe Rookie Posts: 27,747
    Well you explain why it isn't then?

    No external air is in contact with the fluid=sealed.
    External air in contact with the fluid (as on 99.9% of cars)=open system.

    The issue was water absorpotion into the fluid, if no air is in contact with the fluid to carry the moisture it doesn't happen, assuming Osmosis happens (it doesn't to any meaninful level as the materials are not porous or they would leak fluid out as well) then that would be the only way.
  • The RookieThe Rookie Posts: 27,747
    stubs wrote:
    Here you go it took me all of 30 seconds to find this on Google. How hydraulic brakes work
    Except it uses open and closed to describe fixed and variable volume, open is like a car system, open vent to atmosphere, no diphragm, but it's on the internet so it must be true.
  • stubsstubs Posts: 5,258
    I thought you were supposed to work in the hydraulics industry it is car mechanics 101 there are apprentice mechanics whose voice hasnt broken yet know this stuff. Its not sealed or non sealed its open or closed circuit it doesnt matter if the fluid is carried in a bucket on the side of the car. If the car has ABS it is a closed circuit system if its an older car it most likely has an open circuit system. Brakes have seals on the master cylinder whether they are open or closed.

    Now you best hurry off the Millenium Falcon needs some work doing on its sealed plasma cannons that Death Star wont destroy itself you know.

    Keyboard experts :roll:
    Fig rolls: proof that god loves cyclists and that she wants us to do another lap
  • Chunkers1980Chunkers1980 Posts: 8,035
    We weren't discussing open vs closed. We were discussing sealed or not. If something is completely sealed and no room for osmosis then no air/water can get into the fluid.
  • The RookieThe Rookie Posts: 27,747
    stubs wrote:
    I thought you were supposed to work in the hydraulics industry it is car mechanics 101 there are apprentice mechanics whose voice hasnt broken yet know this stuff. Its not sealed or non sealed its open or closed circuit it doesnt matter if the fluid is carried in a bucket on the side of the car. If the car has ABS it is a closed circuit system if its an older car it most likely has an open circuit system. Brakes have seals on the master cylinder whether they are open or closed.
    Open or closed, sealed or non sealed, either way that Epic link uses the wrong terms.

    Cars have no seal above the fluid, it is vented to atmosphere - that is brakes 101, have a look at your car!

    Most cars with ABS have exactly the same master cycliner and resevoir arrangement, even the pressurised ones (like Teves) and vent to atmosphere.

    I don't know where you think the seal is that prevents air entering/leaving the master cylinder on a car, it's what allows the fluid level to drop as the pads wear, in bikes most use a diaphragm above the fluid that isn't in cars.

    Frankly you are talking bollocks.
  • stubsstubs Posts: 5,258
    Wheres that giant facepalm smilie when you need it. Carry On Experting.
    Fig rolls: proof that god loves cyclists and that she wants us to do another lap
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