Disc brake piston position?
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  1. #1
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    Disc brake piston position?

    New to disc brakes on the road and have always taken the mtn bikes to the LBS for brake maintenance. So I'm learning and want to start maintaining the brakes as well now that I have them on my road bike.

    What I'm trying to determine is if the brakes were overbled by the shop who assembled the bike. When the pads removed and the lever not being depressed, are the pistons supposed to be flush with the caliper body or do they normally stick out a mm or so?

    Anyway, I'm trying to move to Ultegra rotors with resin (L03A) on the new bike from the XT (RT81-S) that came on the bike with metal (K04S) pads. The XT rotors and metal pads are screeching to no end. Brake calipers are R8070 flat mount.

    Anyway, when installing the L03A resin pads and the new Ultegra rotors (which are the same thickness as the XT ones) the pads which are thicker, contact the rotor on both sides. I have not squeezed the brakes with the wheels out. I noticed the pistons stick out about 1 mm from the caliper walls and I can't seem to push them back. I'm afraid I will need to open the bleed port to do this - if they are overbled. I cannot find anywhere stating they should be flush or not and wanted to ask before making a potential mistake.

  2. #2
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    The calipers should have an adapter used instead of the pads during bleeding/setting fluid level. Sounds like something is not correctly done.

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  3. #3
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    that's what I'm trying to determine. Do you know if the pistons are supposed to be flush with the caliper body when the pads are out? That is the only way I can see there being enough room for the thicker L03A pads to work.

  4. #4
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    I can confirm that the pistons should be flush w/ the caliper body. I try pushing them back in w/ a tire lever first. If that doesn't get it done I lube the visible part of the piston w/ mineral oil and then CAREFULLY work them back in w/ the box end of an 11 or 12mm combo wrench. Very carefully.
    #promechaniclife

  5. #5
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    The adapter does not push the pistons in until they're flush with the body. Look like it leaves about ~1mm out on either side. This is looking at a NIB GRX caliper.

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  6. #6
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    I always push them all the way back in before resetting. I work on disc brake bikes all day every day. I should have explained in that way.
    #promechaniclife

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    Thanks cxwrench and nova_rider. I will try again with 1st cleaning around the exposed pistons and then trying to press them in flush with a tire lever. If that doesn't work, is it safe to open the bleed port with the funnel and then press them flush?

  8. #8
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    Quote Originally Posted by tinball View Post
    Thanks cxwrench and nova_rider. I will try again with 1st cleaning around the exposed pistons and then trying to press them in flush with a tire lever. If that doesn't work, is it safe to open the bleed port with the funnel and then press them flush?
    You can't bleed fluid out of a Shimano brake that way. It's a closed system so you'd have to push what you think is extra fluid out of the caliper. You can pump the lever all you want w/ the bleed screw out and you won't lose a drop of fluid.
    #promechaniclife

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    Thanks. Wrapping my head around it the whole system concept. I appreciate your patience.

    If I press on the pistons themselves, would that force any excess fluid up to the caliper and out via the open bleed port or would I need to use the lower bleed port on the caliper to pull excess fluid out with the syringe?

    Also do you have a recommendation/preference on a hydraulic hose cutting tool and finisher - preferably one that is applicable to Shimano and SRAM?

  10. #10
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    Quote Originally Posted by tinball View Post
    Thanks. Wrapping my head around it the whole system concept. I appreciate your patience.

    If I press on the pistons themselves, would that force any excess fluid up to the caliper and out via the open bleed port or would I need to use the lower bleed port on the caliper to pull excess fluid out with the syringe?

    Also do you have a recommendation/preference on a hydraulic hose cutting tool and finisher - preferably one that is applicable to Shimano and SRAM?
    Read my last post again. NO fluid will come out of the bleed port at the lever unless you push it from the caliper. If you want to remove fluid from the system it has to come out at the caliper. You can pull it out w/ the syringe or push it out by pulling on the brake lever OR pushing the pistons back into the bores.
    Cutting tools? Lots of good ones, they all use basically a razor blade to cut and have some way to hold the hose. Jagwire, SRAM, or the Shimano BH62 which cuts and also pushes in the barb.
    #promechaniclife

  11. #11
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    Quote Originally Posted by tinball View Post
    Thanks. Wrapping my head around it the whole system concept. I appreciate your patience.

    If I press on the pistons themselves, would that force any excess fluid up to the caliper and out via the open bleed port or would I need to use the lower bleed port on the caliper to pull excess fluid out with the syringe?

    Also do you have a recommendation/preference on a hydraulic hose cutting tool and finisher - preferably one that is applicable to Shimano and SRAM?

    Since it doesn't appear that you wrench for living, you should really consider getting a bleed block shown below to set the proper fluid level and piston spacing rather than eyeballing it. Otherwise, you could end up with lever travel issues if the pistons are too far in.

    51vcZ3-dVSL._AC_SL1200_.jpg

  12. #12
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    Quote Originally Posted by cxwrench View Post
    I can confirm that the pistons should be flush w/ the caliper body. I try pushing them back in w/ a tire lever first. If that doesn't get it done I lube the visible part of the piston w/ mineral oil and then CAREFULLY work them back in w/ the box end of an 11 or 12mm combo wrench. Very carefully.
    as you use your bike and as the pads wear down, don't the pistons must move outward toward the disc? This is due to the fluid pushing the piston outward.

  13. #13
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    Quote Originally Posted by aclinjury View Post
    as you use your bike and as the pads wear down, don't the pistons must move outward toward the disc? This is due to the fluid pushing the piston outward.
    Yes, that's how these work. The OP is trying to reset the pistons to use new pads, but the pistons just do not move inward on their own.

    Sent from my moto x4 using Tapatalk

  14. #14
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    Quote Originally Posted by nova_rider View Post
    Yes, that's how these work. The OP is trying to reset the pistons to use new pads, but the pistons just do not move inward on their own.

    Sent from my moto x4 using Tapatalk
    I guess i'm a bit confused by the OP's question. This was a part of his questions

    What I'm trying to determine is if the brakes were overbled by the shop who assembled the bike. When the pads removed and the lever not being depressed, are the pistons supposed to be flush with the caliper body or do they normally stick out a mm or so?
    I thought he was asking along the line of if it's normal for the pistons to stick out a bit. The answer here is yes it's normal, depending on the initial thickness of pads and/or how much the pads have worn down. Thin pads will have the pistons sticking out some. And one can't "overbleed" the brakes (I got the feeling that OP was thinking somehow one could "overbleed" the brake with extra fluid and this extra fluid would cause the pistons to move outward, which is false, because pistons move outward due to them being thinning down).

  15. #15
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    Quote Originally Posted by aclinjury View Post
    I guess i'm a bit confused by the OP's question. This was a part of his questions



    I thought he was asking along the line of if it's normal for the pistons to stick out a bit. The answer here is yes it's normal, depending on the initial thickness of pads and/or how much the pads have worn down. Thin pads will have the pistons sticking out some. And one can't "overbleed" the brakes (I got the feeling that OP was thinking somehow one could "overbleed" the brake with extra fluid and this extra fluid would cause the pistons to move outward, which is false, because pistons move outward due to them being thinning down).
    No, the pistons move out as the pads wear.

    You can definitely 'overfill' a brake system. You can forget the bleed block, you can bleed w/ pads in and push the pistons out til the pads touch the rotor. I have to fix this **** all the time.
    When I bleed/install new pads I:

    Push the pistons back as far as possible w/ the old pads still in. If they go back all the way great...if they don't...clean/lube then push them back.
    Remove the pads, put in the bleed block. Bleed brake (syringe pushing fluid up to the cup at the lever)
    Put new pads in, use the pad spacer appropriate for your brake, since we're talking Shimano it's the orange one that comes w/ the brake. Pump the lever 10 times.
    Remove the bleeding crap, clean.
    Put the wheel in and pump the lever again. Adjust caliper position as necessary. Don't forget to replace the pad retaining pin clip. Yes...the pistons will most likely be 'out' a bit at this point. I just start w/ them all the way pushed in.

    The 'adapter' nova_rider was talking about is the bleed block. It's job is NOT to push the pistons back, it's just to keep them from getting pushed out of the bores. It's not an adapter. Bleed. Block.
    #promechaniclife

  16. #16
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    Quote Originally Posted by cxwrench View Post
    The 'adapter' nova_rider was talking about is the bleed block. It's job is NOT to push the pistons back, it's just to keep them from getting pushed out of the bores. It's not an adapter. Bleed. Block.
    Well, I see the block as an adapter for the pads, and no where I suggested using the block to push the pistons back. The block does two things: keep pistons in place during bleeding and sets the correct amount of brake fluid. The OP is trying to determine/set the correct piston position for new pads, and the bleed block will do just that. If the OP has to push the pistons beyond what is set by the block in order to use a set of pads, then something is off.

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  17. #17
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    Quote Originally Posted by cxwrench View Post
    No, the pistons move out as the pads wear.

    You can definitely 'overfill' a brake system. You can forget the bleed block, you can bleed w/ pads in and push the pistons out til the pads touch the rotor. I have to fix this **** all the time.
    When I bleed/install new pads I:

    Push the pistons back as far as possible w/ the old pads still in. If they go back all the way great...if they don't...clean/lube then push them back.
    Remove the pads, put in the bleed block. Bleed brake (syringe pushing fluid up to the cup at the lever)
    Put new pads in, use the pad spacer appropriate for your brake, since we're talking Shimano it's the orange one that comes w/ the brake. Pump the lever 10 times.
    Remove the bleeding crap, clean.
    Put the wheel in and pump the lever again. Adjust caliper position as necessary. Don't forget to replace the pad retaining pin clip. Yes...the pistons will most likely be 'out' a bit at this point. I just start w/ them all the way pushed in.

    The 'adapter' nova_rider was talking about is the bleed block. It's job is NOT to push the pistons back, it's just to keep them from getting pushed out of the bores. It's not an adapter. Bleed. Block.
    i meant to say pistons move out as pads thin down, not as pistons thin down (pistons can't thin down because it doesn't come into direct contact with disc).

    your procedure is pretty much what I use. Except I don't use a bleed block. I just leave the old pads in place then push them back as far as possible with a screwdriver, then jam in a piece of wood between the pads.
    Interesting tidbit: My brakes are Magura and it uses a mineral oil called "Magura Royal Blood" (lol). Found out a long time ago this is basically BMW power steering fluid (CHF 7.1) that is 10x cheaper than Magura Royal Blood. Been using BMW fluid in the same Magura brakes for the last 13 years.

  18. #18
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    Quote Originally Posted by aclinjury View Post
    i meant to say pistons move out as pads thin down, not as pistons thin down (pistons can't thin down because it doesn't come into direct contact with disc).

    your procedure is pretty much what I use. Except I don't use a bleed block. I just leave the old pads in place then push them back as far as possible with a screwdriver, then jam in a piece of wood between the pads.
    Interesting tidbit: My brakes are Magura and it uses a mineral oil called "Magura Royal Blood" (lol). Found out a long time ago this is basically BMW power steering fluid (CHF 7.1) that is 10x cheaper than Magura Royal Blood. Been using BMW fluid in the same Magura brakes for the last 13 years.
    I would never bleed a brake w/ the pads in the caliper. At least on a bicycle. On the moto or car yes but the bicycle caliper is small enough and bleed screws are not always located in a good position so there is a good chance you'll get fluid on the pads.
    #promechaniclife

  19. #19
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    Good information from all. Thanks. Nova, I do have a bleed block just haven't done anything bleeding related yet and I do understand it's usage when bleeding. I was just trying to ascertain my starting point and understand the systems better. The bike is new and only has a couple of hundred fair weather miles max on it so pad wear should be minimal. Aclinury is correct in what I was asking. cxwrench, as always, a font of knowledge and experience. I ALWAYS learn something from posting here - I really appreciate it.

    I haven't had much chance to mess with the brakes yet in regards to trying to push out the pistons back into the bores. I hope to get to that this evening after work and will report back if I am able to push the pistons back out for the thicker pads.

  20. #20
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    Quote Originally Posted by cxwrench View Post
    No, the pistons move out as the pads wear.

    You can definitely 'overfill' a brake system. You can forget the bleed block, you can bleed w/ pads in and push the pistons out til the pads touch the rotor. I have to fix this **** all the time.
    When I bleed/install new pads I:

    Push the pistons back as far as possible w/ the old pads still in. If they go back all the way great...if they don't...clean/lube then push them back.
    Remove the pads, put in the bleed block. Bleed brake (syringe pushing fluid up to the cup at the lever)
    Put new pads in, use the pad spacer appropriate for your brake, since we're talking Shimano it's the orange one that comes w/ the brake. Pump the lever 10 times.
    Remove the bleeding crap, clean.
    Put the wheel in and pump the lever again. Adjust caliper position as necessary. Don't forget to replace the pad retaining pin clip. Yes...the pistons will most likely be 'out' a bit at this point. I just start w/ them all the way pushed in.

    The 'adapter' nova_rider was talking about is the bleed block. It's job is NOT to push the pistons back, it's just to keep them from getting pushed out of the bores. It's not an adapter. Bleed. Block.
    Thanks for that great write-up, sticky material right there.
    In reference to the Assault on Mt Mitchell...
    Quote Originally Posted by merckx56
    The easier solution is to find a biker bar in Spartanburg the night before, go in and pick a fight. The ass-whipping you'll get will be far less painful than the one Mitchell will give you the next day!

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