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Shimano BR-M355 rear caliper.

I've very quickly managed to contaminate three sets of brake pads through fluid leaking (I think) from the bleed nipple. I've twice taken advice from local bike shop who inspected the caliper and assured me nothing was wrong, advice was to buy more premium end sintered pads. Fair enough I thought, give it a try. Few weeks later and back to square one - no braking power and very noisy.

The dust cap is missing from the bleed nipple, but I assumed this was one way and wouldn't cause it to leak?

Essentially, the question is can I fix it by replacing the dust cap, top up fluid and replace pads, or is it a case of replacing the caliper and starting again?

Any advice gratefully received. Thanks

  • Somebody tampered with the bleed nipple? Missing cap could be clue. – Carel Jul 14 at 11:50
  • This is a common failure method of Shimano brakes. The seals around the pistons degrade then leak. – DWGKNZ Jul 14 at 20:09
  • Interesting, thanks and good to know. In fairness they've had use - 10,000 miles or thereabouts. – Tom Mayfield Jul 14 at 20:11
  • Probably the best fix - quick, reliable and relatively cheap is a new caliper (or new brake set). Pricewise, how much have you already spend on new pads and how long before you give up throwing good money after bad? – mattnz Jul 15 at 1:09
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Are you 100% sure that it is leaking from the nipple?

There are other ways to leak oil:

  • From a badly crushed olive in the hose connection
  • Cracked piston
  • Damaged piston seal

And other issue could be "pad glazing". This has similar symtoms as getting oil on the pads.

If you are 100% sure that the bleed nipple is the issue. You can just replace and bleed the brakes.

You can't just get a new dust cap though. You would have to get a new bleed nipple. The dust cap has nothing to do with making sure the system doesn't leak. It's purpose is to keep dust out so when you bleed the system, the dust doesn't contaminate by mixing dust in with the oil. Adding a dust cap, won't stop a leak. The pressure from braking will still push the oil out.

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  • Thanks, kind of confirmed what I suspected that the missing cap is not the problem. I'm not 100% on the bleed nipple, its just the fluid leak appears to be coming from there. Not confident with working on hydraulics so haven't investigated myself, can only go on what bike shop have told me, which is there's "nothing wrong" – Tom Mayfield Jul 14 at 12:47
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First off, the metallic pads were a clear upsell. I'd be wary of that shop. As a side note though, metallic pads will require more force than resin pads before they bite, but will aggressively bite (noticeably more powerful than resin) once you reach that threshold. The metal-on-metal contact also means they're noisier, especially in the rain.

Is this a new bike? When it comes to "it worked previously but doesn't anymore", you need to figure out what changed between then and now.

Can you see evidence of oil leaking from anywhere? Make sure the bleed nipple is tight.

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  • Yes, as already mentioned there is clear evidence of fluid coming from the bleed nipple area. Not a new bike, 5 years old and had alot of use. The shop didn't sell me the metallic pads, just offered advice. clearly wrong in this case as there's obviously a leak but I take your point about going elsewhere. It's a relatively low end shimano caliper so replacement seems easier than repair at this stage – Tom Mayfield Jul 14 at 19:42
  • @TomMayfield I would try tightening the nipple some more and hoping for the best. At worst you'll just need to buy a replacement which you were already planning on doing. Heck, permanently glue the whole area shut and just replace it next time you need to do a bleed. – MaplePanda Jul 14 at 20:17

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