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Thread: Shimano Master Cylinder Failure After Bleed?

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    Default Shimano Master Cylinder Failure After Bleed?

    Brake in question is a Saint M820. I've had them since 2014; never really had to bleed them other than purging a few bubbles when I moved them to new bikes and had to shorten hoses.

    Last season my lever would start randomly pulling all the way to the bar, so I figured it was time to flush the old oil and do a full re-bleed. Two weeks ago I did this full bleed. The oil was extremely dirty. I was pushing clean oil from the caliper up to the master cylinder, which is probably not ideal for keeping contaminants out of the master cylinder, but that's the way all the bleed kits are designed to work. Pushed two syringes worth of oil through and then did a proper bleed. Brake felt great after the bleed. Got two rides in since the bleed; lever felt nice and firm, no wandering bite point, no pulling to the bar randomly.

    Today I was tooling around the parking lot and noticed the brake lever in question was not snapping back as fast as the other brake lever. Probably like 1/2 a second of extra time for the master cylinder pushrod to retract and push back on the brake blade. I started looking at the cam roller to see if it was dry/hanging up, and i swear the master cylinder completely failed while i was holding it in my hands. The lever will depress the cylinder, but it only puts a marginal amount of pressure on the pistons and it also stays depressed. I have to use a pick to grab onto the roller and get the pushrod to retract.

    I did a quick re-bleed to see if there was some sort of massive bubble in the caliper. I pushed another syringe worth of oil from the caliper up to the master cylinder. It was once again black. No change in the performance of the lever.


    Anyone seen this before? The thing is basically fucked. I'm used to seeing a little discoloration in old brake oil, but this looks like some seal has completely disintegrated and dissolved into the oil.
     

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    Default Re: Shimano Master Cylinder Failure After Bleed?

    You always have to pull oil down. Fill the cup and let all the black stuff drain out. You start with a mostly empty syringe and fill it until the oil is clean. You might have pushed material up into the master cylinder and scored the piston walls.
     

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    Default Re: Shimano Master Cylinder Failure After Bleed?

    Have seen this plenty of times. Pushing old fluid up and pushing dirty pistons in before cleaning is asking for issues. I would keep trying to get the black fluid out through the caliper. Do you see signs of fluid leaking out behind the lever piston? Not the port on the end of the lever (gold plug on saint)

    Can you cap the lever and push some fluid up from the caliper to pressurize the lever and see if it pops the lever out?
     

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    Default Re: Shimano Master Cylinder Failure After Bleed?

    Quote Originally Posted by andrew flowers View Post
    You always have to pull oil down. Fill the cup and let all the black stuff drain out. You start with a mostly empty syringe and fill it until the oil is clean. You might have pushed material up into the master cylinder and scored the piston walls.
    This should be in bold letters somewhere. I read the instructions in the dealer manual, and only on my second or third reading, did I realize that the bleed direction is down through the caliper - which makes sense because that's the way hydraulic brake systems work in other vehicles. But for whatever reason (my brain, their brain, vague drawings, language in general) it wasn't initially clear to me that those were Shimano's instructions as well.
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    Default Re: Shimano Master Cylinder Failure After Bleed?

    Quote Originally Posted by j44ke View Post
    This should be in bold letters somewhere. I read the instructions in the dealer manual, and only on my second or third reading, did I realize that the bleed direction is down through the caliper - which makes sense because that's the way hydraulic brake systems work in other vehicles. But for whatever reason (my brain, their brain, vague drawings, language in general) it wasn't initially clear to me that those were Shimano's instructions as well.
    Sounds like my brain was wired wrong as well. I played around with the lever some more and its totally dead. Lesson learned.
     

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    Default Re: Shimano Master Cylinder Failure After Bleed?

    Quote Originally Posted by Frightened Inmate #2 View Post
    Sounds like my brain was wired wrong as well. I played around with the lever some more and its totally dead. Lesson learned.
    luckily you broke the cheap part of the system.

    pushing up is a valid move, but you NEED to ensure that all worn fluid has been drained from the system

    I believe applesauce suggested gravity bleeding the unit dry first, which isn't a bad idea if you are doing it at home and have the time for a longer bleed process.
     

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