I put an old set of shimano BR-M575s on my commuter bike a few weeks ago. The rear brake was particularly spongy / ineffective so I bled them 7-10 days ago. At the time it seemed OK. a couple of hours later and it wasn’t quite as good as I thought I remembered. I’ve not been on the bike for a week but this morning it is back to being as bad as it was before I started (lever travels full way before any braking starts). There is no visible sign of an oil leak. The bleed points & the hose all seem to be tight…

…is there any simple way to trace a small leak?
…are there any common / known places to look for?

The brakes will be 5-ish yrs old but have been sitting unused for 2+ yrs.

Rotten piston seal? If they have been sat not used for a while the seals may have perished

We used to have a lot of problems with 575s needing frequent bleeding, never any obvious signs of leaks, but always full of air within a week or two of bleeding.Most of them were replaced under warranty, but not much use with the age of yours, i’d probably just replace them with a bargain set of 615s, almost as cheap as buying new levers or calipers in an attempt to salvage yours.

That could just be a bad bleed- sometimes it’s just air settling/going back where it shouldn’t. Generally you should be able to find a leak, since it’s much easier to push things out with pressure than it is to have air leak in through a hole…But not always, especially if it’s the lever.

That could just be a bad bleed- sometimes it’s just air settling/going back where it shouldn’t. Generally you should be able to find a leak, since it’s much easier to push things out with pressure than it is to have air leak in through a hole…But not always, especially if it’s the lever.

So you suggest “rebleed” and try again – but it sounds like if that doesn’t work its replace the whole thing…

What fluid did you use?

Dirty dog -mineral oil (overpriced bike specific stuff as LBS is closer than the motor factors!)

Just checking 😉

Wasn’t Magura was it?