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  1. #1
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    Shimano SLX 665 brake question

    Lately I've noticed that when I set the lever reach to where I want it and ride hard (using brakes a lot), the lever position is much firmer (less lever throw) to operate the brakes.

    What's this a sign of? Old fluid? Rebleeding? Never acted up like this in the past 2 years I've owned them.

    Thanks!

  2. #2
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    Mineral oil overheating.

  3. #3
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    Quote Originally Posted by skaktuss View Post
    Mineral oil overheating.
    Due to being old?

    Guess it's time for rebleed and new oil then. First time bleeding brakes here I come.

  4. #4
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    Eurospek, it's simple physics.

    Using brakes a lot = more friction between rotor and pads -> more heat to pistons -> more heat to mineral oil -> more kinetic energy is added and molecules run faster, thus increasing oil volume and pressure in system.
    Last edited by skaktuss; 11-29-2012 at 11:21 PM.

  5. #5
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    Quote Originally Posted by eurospek View Post
    Lately I've noticed that when I set the lever reach to where I want it and ride hard (using brakes a lot), the lever position is much firmer (less lever throw) to operate the brakes.

    What's this a sign of? Old fluid? Rebleeding? Never acted up like this in the past 2 years I've owned them.

    Thanks!
    Are you saying they occasionally have less free stroke during the ride? If so, mine did this recently as well. They always did it when riding in the cold, but it started happening under normal conditions. Less free stroke means the pistons aren't returning. I pulled the calipers & pads, extended the pistons, cleaned w/mineral oil and reassembled. They've been fine since.

  6. #6
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    Quote Originally Posted by skaktuss View Post
    Eurospek, it's simple physics.

    Using brakes a lot = more friction between rotor and pads -> more heat to pistons -> more heat to mineral oil -> more kinetic energy is added and molecules run faster, thus increasing oil volume and pressure in system.
    But would fresh mineral oil kinda help the situation from happening again so soon? I haven't had new fluid in 2+ years.

    Quote Originally Posted by car_nut View Post
    Are you saying they occasionally have less free stroke during the ride? If so, mine did this recently as well. They always did it when riding in the cold, but it started happening under normal conditions. Less free stroke means the pistons aren't returning. I pulled the calipers & pads, extended the pistons, cleaned w/mineral oil and reassembled. They've been fine since.
    Yes. Well this started happening when it's under 50F outside, so might as well the pistons might be sticking, I don't have any Shimano mineral oil to lube them up, will be ordering a bleeding kit tonight.

  7. #7
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    Quote Originally Posted by skaktuss View Post
    Eurospek, it's simple physics.

    Using brakes a lot = more friction between rotor and pads -> more heat to pistons -> more heat to mineral oil -> more kinetic energy is added and molecules run faster, thus increasing oil volume and pressure in system.
    The volume increase in mineral oil from a temperature change is pretty small. This volume change is easily accomodated for by the rubber diaphragm in the master cylinder. Boiling the fluid or the moisture absorbed in the fluid is a whole different story, but that would have presented with other issues that he didn't mention.

    Quote Originally Posted by eurospek View Post
    Yes. Well this started happening when it's under 50F outside, so might as well the pistons might be sticking, I don't have any Shimano mineral oil to lube them up, will be ordering a bleeding kit tonight.
    My cold weather issues don't present until it's at or just above freezing. Try cleaning them up and see how it goes. Just be careful with how much oil you put in there. Nothing short of brake cleaner seems to really remove mineral oil and it seeks out braking surfaces like a hound dog.

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