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Is this common on disc brakes?

Old 10-02-22, 11:47 PM
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MarcusT
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Is this common on disc brakes?

I was bleeding the hydraulic disc brakes for Shimano BR-MT520. I placed the block in the calipers, pulled the lever and after half a second the pressure released then the caliper started leaking fluid. I know it can happen that a seal or piston will wear down, but there is only a few thousand kms on this bike.
Did I do something wrong?
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Old 10-03-22, 09:22 AM
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If you used the yellow bleed block there shouldn't be any way this could happen. You had the pads out of the caliper and used a yellow block? Or...?
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Old 10-03-22, 10:01 AM
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Yes, pads out, yellow block in
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Old 10-03-22, 03:06 PM
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Did you push the pistons in? What tool did you use?
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Old 10-03-22, 04:57 PM
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Did they leak when you're just riding the bike?
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Old 10-03-22, 10:07 PM
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Where exactly...and I mean exactly...was the leak?
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Old 10-03-22, 10:09 PM
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They worked fine before, pistons were pushed back with with a plastic tire lever. I took the calipers apart and found when I pushed the leaking piston in, there was a crunching sound.
I guess with the lack of answers that this may have just been a fluke. Well, the replacement calipers arrive today. Hope it goes well
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Old 10-03-22, 10:43 PM
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I suspect that when you somehow pushed the pads out farther than normal, the seal picked up some dirt sitting outside it's normal working range, and then dragged it back where it interfered with the seal.

Since you're ready to declare these dead and have a replacement on the way, it's time for an autopsy. Take the brake apart, see if you can figure out what happened, and who knows, you might even be able to fix it, and have a spare. The worst that happen is that you learn something.
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Old 10-04-22, 10:50 AM
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My best guess, is the block was not long enough. (Yes, a chinese clone, not original Shimano) This is a 4 piston caliper and even though the block was in place with the pin, I'm afraid it may have moved so half a piston was not "blocked" so when hydraulic pressure applied, the piston came out tilted, thus shattering the ceramic.
Live and learn. The caliper was not stupidly expensive, so a costly lesson.
Thanks for the input, hopefully my error will aid others in the future
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Old 10-04-22, 03:48 PM
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Originally Posted by MarcusT View Post
They worked fine before, pistons were pushed back with with a plastic tire lever. I took the calipers apart and found when I pushed the leaking piston in, there was a crunching sound.
I guess with the lack of answers that this may have just been a fluke. Well, the replacement calipers arrive today. Hope it goes well
So the leak was at a piston, not the caliper halves or the bleed port or the point where the hose attaches?
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Old 10-08-22, 10:32 AM
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Originally Posted by MarcusT View Post
I took the calipers apart and found when I pushed the leaking piston in, there was a crunching sound.
Did you actually dismantle the calipers, or did you just push the pistons back?
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