If I dribble water from my camelbak on the brakes (Shimano SLX old style) I get a decent bit of power and can lock up if necessary, but it goes away quite quickly and when dry no power, difference between endo with one fingure and deceleration with a handful of brake.
glazed / contaminated pads would be my bet
Yeah, change your pads
again, one day I will wear out some pads, have bought shimano this time, called time on Superstar for now. I have cleaned with meth, sand paper, etc. pads and rotors so many times but seems the contamination keeps returning. I'm not even being fast and loose with WD40 etc. In fact i'm the opposite.
tiny leak from your caliper / pistons?
I am coming to that conclusion, is it servicable, as far as I can see that parts available are a new caliper, a new lever and a new hose, no seals or pistons etc. although I'd probably not want to strip it down anyway. Fed up of inconsistent braking though.
No spares available for calipers levers, specifically seals.
Have there been any changes in lever "feel"? Have the brakes been bled recently?
pnik. I am beset with the same problem. My only conclusion is that it is my bike-washing technique. I think I must spray oil water off the chain on to the rear disk. Despite being very careful and despite not having a leaky calliper I regularly have to clean my disk, bake the rear pads, sand them down a little and re-bed them. All is then well...until the mystery contamination strikes again.
Maybe you could rig up one of those Scottoilers to water your calipers?
Seriously though, that must be really annoying.
Yes annoying it is, I had an over the bars moment on Saturday at Swinley when the brakes unexpectedly worked properly. Lots of water around.
I think there may have been a gradual loss of fluid, although so small that I've only just started to seriously consider a leak as the cause. I was singing the praises of these brakes for the first 2 years, but of late they've been a pain.
I like the scottoiler idea. Seems I am not alone with this, so much for fit and forget.
My few-year-old XT calipers both leaked. You couldn't see them doing so, I couldn't tell a difference in lever feel but every so often the pads would stop working very well and squeal a lot until I put some new ones in. After replacing both calipers they have been fine since. I put this down to the bike living in an uninsulated shed over the past two super-cold winters; I think the seals decided it was just too damn cold and gave up the ghost...