Shimano SLX M675 leak from level cap (master cyclinder)

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Shimano SLX M675 leak from level cap (master cyclinder)
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Posted: May 17, 2016 at 11:41 Quote
I have a Brand new set of Shimano brakes that had the hoses shortened and bled by my local bike shop. I was then injured in a non biking related incident and unable to ride for a year. Therefore, I never really had a chance to test out the brake install and bleed. Now, a year later, the rear brake seems spongy. I then attempted to bleed it myself.

1st time around, brake felt the same. Just inconsistent level pull, sometimes firm, sometimes spongy.

2nd time around, same feeling, but this time I noticed what seems to be a tiny tiny leak coming from under the cap at the lever. I am unsure if this was a result of just excess fluid above the diaphragm being pushed out or if it is an actual leak. So I took everything apart to inspect, found nothing out of the ordinary, reassembled and bled again.

3rd time around, still had some leakage, however the lever pull seems to be more consistant. Cleaned everything up and went to bed. I get up in the morning, walk downstairs and proceed to squeeze the lever, fast, slow, hard, soft, for about 3-5 minutes. NOT ONE DROP and consistent lever feel. Why all the leaking the night before then?

My question I guess, have you ever seen this before? Shimano suggested it was the separator unit assembly that sits above the diaphragm. Other forums suggest it could be a master cyclinder oring leaking, and since this is an open diaphragm brake, the fluid is leaking back out into the reservoir and over flowing.

Is this worth chasing down and bleed again or trying the new separator assembly or do I just look at replacing the whole brake lever?

Any assistance is greatly appreciated.


Posted: May 20, 2016 at 9:31 Quote
My Deore brakes with the same lever design would leak slowly for a week or two every time I bled them and then would quit. I checked fluid levels months later and no issues. I'm into my third season on these brakes and they're flawless. It's just overfill leaking. Give it a few rides, clean it, then check the fluid level. As long as there's enough in there and the levers feel good, everything is working fine.

Posted: Sep 19, 2020 at 19:21 Quote
I also noted this issue with a bit of oil bubbling out and leaking from the cylinder caps after bleeding the brakes. It seems to have resolved itself after 5-6 hrs of use.

It doesn’t make a lot of sense to me, I took apart the whole cylinder top cap and put it back together and rebled the system about 4 times before giving up on it and just riding it. The issue seems to be gone.

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