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I have the Park tool mineral oil bleeding kit (BKM-1), and followed their instructions for bleeding my MT5 calipers.

I ended up with soft/useless brake caliper. I think maybe I introduced air into the system when I took too long to insert the stopper bolt on the caliper-side, and some oil tumbled out. (as an aside -- with the lever-side bleed screw closed, I thought the sealed system would prevent oil from tumbling out the caliper-side port, similar to how I can use a straw and my finger to lift coke out of a glass. But it doesn's seem to be the case here)

So I went back and repeated the process, adding more fluid in via syring/funnel combo and trying to drive the air out. I got it to the point that I didn't have any big glug-glug bubbles, but I seemed to have an incessant plume of extremely fine microbubbles on the caliper side when I sucked with the syringe, and on the funnel-side when I pressed the syringe. I was working at night with a head-lantern, so maybe this was overly sensitizing me to the microbubbles, but they steadfastly persisted. It's like I somehow atomized the air and diffused it so widely in the fluid it was never going to come out, or at least not till they had a long time to aggregate into larger bubbles?

incessant fine bubble stream

I had the fittings all snugged on as tight as my fingers allowed and they had their rubber gaskets, so I don't see where there might have been a leak that could suck any air in.

I eventually just pushed up into the funnel, and replaced the fluid with fresh fluid that presumably didn't have all this atomized air in it. Once I pulled that in and sealed up the system the brakes work, at least ok. (I'm not sure if they're as strong as they could be -- I couldn't get my front to lock-up.)

One variation from the Park guide that I did my second time was detach the caliper from the fork and bring the vent hole up to parity with the lever-side vent so that I didn't leak oil or leave space when I put the stopper bolt on. That seems to have helped.

Anyway, my question is, "what was I doing wrong to generate all these microbubbles which are the source I my problem I assume?"

And I guess secondly is there a followup process I could do to further purge any remaining air from the system?

Thanks

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  • At the end of the day, in situations like this you need to consider that the third party kit instructions are essentially made-up and arbitrary. Try using it to recreate the official Magura procedure and go from there. Push oil up from the caliper into a collection bat at the syringe and see what you get. (If I recollect correctly they have a consumer and a pro kit available, and this is how the consumer one works for all models.)
    – Nathan Knutson
    Commented Jun 16, 2021 at 18:51
  • Did the hose/tube ever turn back downward? I have found that its best to make the hose descend smoothly without any ups and downs. And to let the air move up into the lever might take a while. Brake bleeding is not a quick process at first.
    – Criggie ♦
    Commented Oct 14, 2021 at 11:05
  • @NathanKnutson first time I've ever heard anyone suggest that any maintenance advice from Calvin Jones was either made-up or arbitrary! I think the issue is likely the OP didn't fully purge the air out when creating a vacuum with the syringe. Brake bleeding is an iterative process and worth doing over and over until you master it.
    – DWGKNZ
    Commented Oct 14, 2021 at 12:58
  • Not beeing able to lock the front wheel when you only see those small bubbles is not a good sign. Is it possible you contaminated rotor or pads? And 'snugged on as tight as my fingers allowed' - be very careful with the plastic screw in your brake lever!
    – reciprocal lettuce
    Commented Sep 15, 2022 at 14:11

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I thought the sealed system would prevent oil from tumbling out the caliper-side port, similar to how I can use a straw and my finger to lift coke out of a glass. But it doesn't seem to be the case here

I had similar experience with Magura brakes — just shutting the lever-side port closed did not stop oil from leaking from the caliper body's opening.

One variation from the Park guide that I did my second time was detach the caliper from the fork and bring the vent hole up to parity with the lever-side vent so that I didn't leak oil or leave space when I put the stopper bolt on. That seems to have helped.

Raising the caliper body at the lever's level is what the official Magura video tells us to do; see timestamp 3:30.

And I guess secondly is there a followup process I could do to further purge any remaining air from the system?

When I had problems with banishing of remaining bubbles from my Magura calipers after a full bleed process, a follow-up quick bleeding process helped me. See the official video:

Because only the lever's port gets opened during this process, there are no dangers of introducing new air to the system via the bottom hole.

I used Magura's own bleeding kit with a syringe with a hole at 30 ml mark to achieve so. I do not know whether Park Tool's kit that you have has a matching device or not. If nothing else, you can make it yourself from a regular 30 ml syringe by punching a hole with a needle.

Good luck!

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