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  1. #1
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    Overpressurizing fluid equals less pad-rotor gap?

    Does anyone know if over-pressurizing the fluid in the brake lines when bleeding (Avid Juicy or Elixir) results in little or no gap between the pads and rotor?

    It seems like all the disc brake builds on new bikes at LBS's seem to have more gap and no pad rub when free spinning the wheels. How is that accomplished?
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  2. #2
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    If you overfill the system with fluid then there will be less pad gap.

    Be sure you're resetting the pistons before bleeding. Or level out the lever, open the bleed port on the lever, reset the pistons. This will force out any excess fluid.

    Also make sure your caliper is centered.

  3. #3
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    Using excess fluid also makes the brake more suseptible to temperature changes, in terms of rubbing and long throw, alternately. It also makes the brake more suseptible to pump up when it gets hot.

  4. #4
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    So does it make sense to bleed the brakes with wheels/rotors on.....insert business cards or something that creates the desired gap, then bleed the system?
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  5. #5
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    "So does it make sense to bleed the brakes with wheels/rotors on...."

    Leaving the wheel/rotor in place creates two potential problems: one is over-filling the system, the other is getting brake fluid on the pads or rotor. Removing the wheel and then pushing the pistons back into the caliper so that the face is flush with the caliper interior is the basic set-up for a good bleed. I use a cut-to-fit block of wood pressed in between the pistons to ensure that they remain static.

    When you first pull the brake lever after a bleed you'll find that you need to do so a few times until the lever becomes firm and the pads bite the rotor properly. This process transfers fluid from the master cylinder (MC) to the 'working' part of the brake system. The space created in the MC (by the fluid which now resides behind the pistons in the caliper) allows the system fluid to expand from the heat of braking without causing the pistons to over-extend (and thus causing the pads to rub).
    Last edited by SteveUK; 10-27-2008 at 05:38 PM.
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  6. #6
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    I flubbed a bit with my wording on the "alternately" part: I meant to say that on some days, it can have a standard or longer throw, or on warm days, the brake can have little throw and the pads rub against the rotor as the temperature causes the fluid to expand while there's an excess in the system.

    Sorry about that miswording.

  7. #7
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    Good tip

    The wood block thing makes a lot of sense, nice tip. Fortunately, Elixirs have a nice easy access from the top into the pad/piston area, so making some kind of bleed block should be easy.

    Kind of kicking myself now after all these years dealing with constant pad rubbing, assuming that was the norm.
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    No need to make a bleed block for the Elixirs because Avid supplies one with the brake. I just installed a set and the red spacer that is inserted during shipping to keep the pistons separated also acts as a bleed block when inverted.

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    You're right, the spacers are also bleed blocks. Still rubbing on the front calipers, might have to re-bleed the system and not pressurize the line this time.
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    "Still rubbing on the front calipers, might have to re-bleed the system and not pressurize the line this time."

    I know it might seem like I'm splitting hairs, but in the interests of integrity it should be pointed out that the system is not pressurised, at least not until the brake lever is pulled. I think, by "over-pressurise" you actually mean over-fill.
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  11. #11
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    Correct, 'overfill' is probably more accurate. We'll see how it goes, because I did use the bleed blocks and followed the bleed procedure closely. Got the rear to spin freely by sucking a bit of fluid out from the lever, but the front didn't seem to make a difference.

    The other thing I'm waiting for is a Goodridge brake line kit for Elixirs. They made such an improvement to my prev Juicy's, getting rid of the spongy feel at the lever.
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  12. #12
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    Quote Originally Posted by HTail
    The other thing I'm waiting for is a Goodridge brake line kit for Elixirs. They made such an improvement to my prev Juicy's, getting rid of the spongy feel at the lever.
    It was more likely due to a need for complete bleeding than the hoses. There isn't a single brake I've used or converted that felt different with SS braided hoses.

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    At least with Juicy 7's, the Goodridge lines felt like the difference between sucking a milk shake from a thin straw versus a large one...ok, that's a bad analogy Seriously, prior to Goodridge lines, a spongy lever feel, after, snappy/positive feel.

    Still looking for a set on my Elixirs.
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  14. #14
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    Quote Originally Posted by SteveUK
    "Still rubbing on the front calipers, might have to re-bleed the system and not pressurize the line this time."

    I know it might seem like I'm splitting hairs, but in the interests of integrity it should be pointed out that the system is not pressurised, at least not until the brake lever is pulled. I think, by "over-pressurise" you actually mean over-fill.
    granted, you can't over pressurize an open system since its pressure is equal to the atmospheric pressure

    ***IMPORTANT*** when your pads are half way used or more, backing out the pistons would not be a good idea since you would be missing fluid in the system when the pads would have to compensate for the small amount of brake pads

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    "***IMPORTANT*** when your pads are half way used or more, backing out the pistons would not be a good idea since you would be missing fluid in the system when the pads would have to compensate for the small amount of brake pads"

    This is not correct. As I've already explained to you once today, re-filling the system with the pistons partially extend will effectively over-fill the system, leading to problems of pump-up when the brake is used a lot. The master cylinder is designed to have sufficient capacity to allow it to retain enough fluid even when the pads are worn down to the plate, and also to have sufficient free space for fluid expansion when the pads are new.
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  16. #16
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    Quote Originally Posted by SteveUK
    "***IMPORTANT*** when your pads are half way used or more, backing out the pistons would not be a good idea since you would be missing fluid in the system when the pads would have to compensate for the small amount of brake pads"

    This is not correct. As I've already explained to you once today, re-filling the system with the pistons partially extend will effectively over-fill the system, leading to problems of pump-up when the brake is used a lot. The master cylinder is designed to have sufficient capacity to allow it to retain enough fluid even when the pads are worn down to the plate, and also to have sufficient free space for fluid expansion when the pads are new.
    my bad, forgot that the tech site is yours and just checked the reply. So let me get this straight

    1) by putting too much oil in your system by bleeding and not retracting the pistons....you get pump up...i've heard that term soo many time and not figured out exactly what it meant...in my perspective pump up would be the system of a brake that news to be applied many times to work...if possible plz extrapolate in precision about "pump up"

    2) by having worn out pads and pushing the pistons back in, there will obviously be a certain amount of air around your master cylinder...as you metionned this "free space" allows the fluid to expand...this being said, what about if you had SS goodridge lines that claim there are "zero expansion of the lines"...maybe i'm confusing fluid expansion and actual hose flex...

    3) would it be better to use a hayes/avid bleeding technique for all brakes considering the positive natural buoyancy of air, wanting to always escape upwards??

  17. #17
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    I'm at work here so this might be a little rushed and occasionally interupted, but I hope that this will help answer all three questions for you...

    You understand that DOT fluid will expand when it is heated? And that brake fluid can be heated as part of the braking process (which uses friction (pads to rotor) to convert kinetic into heat energy)? The main 'working' part of an open hydraulic system is the brake line, and the section of fluid between the back of the caliper pistons and the front of the MC piston (seal). When a brake system is first installed, the pistons are pushed completely back into the caliper and the lever is in the 'rest' position. This can be considered the default position of the brake. Now, the purpose of a master cyclinder is two-fold. One is to store the fluid which is required to fill the system as the pads wear, the other is to allow for expansion and cooling of brake fluid should it be heated during the braking process.

    In order to acheive both of these aims, there must be a specific amount of fluid within the whole system, and also enough space - not air, space.

    You need to begin your visualisation from the brake's default position of pistons back, MC full and pads whole. When the lever is first operated at installation, it generally takes two or three pulls to get the correct throw and feel. During this process, the pistons extend from the caliper far enough to push the pads against the rotor. This is all intentional as by extending the pistons, and thus filling the space behind them with fluid from the MC, a space is created within the MC. If I understand this correctly, within the actual working system, this space is virtual. The air space exists on the outside of the MC diaphragm, between the diaphragm and the MC cap. The space in the actual system is filled by the diaphragm being pulled inwards. As the pads wear, the pistons have to extend further and further to press them against the rotor, while all the time the diaphragm is compensating for this change (increase) in volume. If at any point in all of this the fluid expands from heating, it does so into this virtual space in the M and the diaphragm rectracts outwards, expelling the air above it through the pin-hole one will invariably find at the MC cap.

    Now, if at this point the MC cap is removed, the system is bled and refilled with fluid, the volume of fluid will be greater than the default, because the pistons are positioned further out of the caliper. When the MC is replaced, the system is completely full, with no virtual space for expanding fluid to push into. The only way that the system can deal with this expansion is to push the pistons further still out of the caliper, and thus jamming the pads against the rotor. You're now experiencing 'pump-up'.
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  18. #18
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    Quote Originally Posted by SteveUK
    I'm at work here so this might be a little rushed and occasionally interupted, but I hope that this will help answer all three questions for you...
    great well this explains the pump up...but still i have 2 questions:

    1) the bleeding technique i mentioned before...dispelling air upwards
    2) the goodridge claim of no expansion

  19. #19
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    "1) the bleeding technique i mentioned before...dispelling air upwards"

    The procedure for bleeding Hopes is about as complicated as it should get, in my opinion. Some folks use a syringe to push DOT fluid from the caliper, but they're basically just adding an extra tool to do what the MC lever will do for you and requiring yet another tool to collect the spilling fluid from the open MC. At least with the standard method, a pipe can be easily and cleanly fitted to the bleed nipple.

    "2) the goodridge claim of no expansion"

    This isn't a claim about the fluid, it's a claim about the actual hose. Standard hose has a nylon braid to help prevent the plastic tube portion of the hose from expanding outwards under the pressure caused when the lever is pulled. Goodridge uses a steel braid which, it is claimed, does a better job of preveting the plastic tube expanding.
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  20. #20
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    Quote Originally Posted by SteveUK
    The space in the actual system is filled by the diaphragm being pulled inwards.
    More picking of your brain, what about brakes that do not have a "reservoir type" lever like the hopes,juicy's,maguras? how would their sleek look handle fluid expansion?

    also, according to the magura tech, every open system has either a little hole or a slid allowing the "reservoir" or diaphragm to be at equal atm pressure as the outside pressure

    this would mean in essence, as i mentioned, that the hose is never really pressurized in neutral position...

  21. #21
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    Quote Originally Posted by SteveUK
    "1) the bleeding technique i mentioned before...dispelling air upwards"

    The procedure for bleeding Hopes is about as complicated as it should get, in my opinion. Some folks use a syringe to push DOT fluid from the caliper, but they're basically just adding an extra tool to do what the MC lever will do for you and requiring yet another tool to collect the spilling fluid from the open MC. At least with the standard method, a pipe can be easily and cleanly fitted to the bleed nipple.
    but in theory, would you agree that bleeding from the bottom would potential hinder better results?

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    "More picking of your brain, what about brakes that do not have a "reservoir type" lever like the hopes,juicy's,maguras? how would their sleek look handle fluid expansion?"

    Quite badly, I would imagine. Which systems still use a closed system?

    "also, according to the magura tech, every open system has either a little hole or a slid allowing the "reservoir" or diaphragm to be at equal atm pressure as the outside pressure"

    Yes, I mentioned the pin-hole at the MC cap in my previous post. Perhaps I've just slightly misunderstood the action of the diaphragm in the whole process? There must, however, be some sort of pressure acting on the diaphragm at some point, otherwise air would be drawn into the line when the pistons were pumped out at install, and then also as the pads wore down. I see it as changing shape, even if it's not actually held in that shape by a small vacuum pressure in the MC (although not actually in the line itself). Maybe we'll get an interjection from somebody who can explain precisely how this part of the system contributes to the system as a whole?

    I'm pretty much entirely self-taught through observation, so I wouldn't be surprised if there are holes in my understanding. I'm not the smartest bloke you'll ever meet.
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    "but in theory, would you agree that bleeding from the bottom would potential hinder better results?"

    I don't see how one is better than the other, but as I said, the system alreday has a lever at one end anyway, it makes sense to just fit an outlet (bleed nipple) at the other.
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  24. #24
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    So to recap on the original topic, to achieve proper 'gap' between the pads and rotors - the pistons need to be recessed or flush to the caliper walls when bleeding to set the gap?

    I guess in my bleeding experiences so far, the pistons seem to have some default resting point regardless of the fluid level- is this not true? In other words if I take fluid out of the system, and push the pistons in, does not equal more gap....or do I have to re-bleed the system and the last step push the pistons in further than the bleed blocks, allowing some excess fluid to seep out?

    Again, every time I play with LBS disc brakes, the pad travel is visually noticeable, on my bike the pad hardly moves before contacting the rotors.
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    "I guess in my bleeding experiences so far, the pistons seem to have some default resting point regardless of the fluid level- is this not true?

    The pistons will extend as far as the lever tells them and retract only as far as the piston seals allow (do you understand the role that the piston seals play in the actuation of the brake?). As long as there is no air in it, the fluid in the line simply transfers pressure from the lever to the pistons/pads. When bleeding a brake, we need to take the system capacity to its minimum and fill that to its absolute maximum to ensure that no air is present.
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