here's an interesting issue that my neighbour is having with his shimano hydraulic disc rear brake. Since last year he mentioned that the lever sometimes would go to the handlebar (hybrid bike, so straight type bar) but then be ok. I looked at it and then thought maybe it was because he hangs his bike from a hook in his garage, so I thought maybe an air bubble was moving somewhere from the resevoir , I thought he hung it bars down.
Thought this was the issue, as it worked fine after being kept on two wheels, but recently he told me its doing it again , lever to bars and little braking.
I tried it and sure enough, I couldnt lock the wheel. The pad material was thin but still ok, from my experience with my mechanical discs I found his pad material thickness alright, not great but ok.
My concern was fluid loss, so I told him to look all around for any sign of leakage, either wet or even just lots of accumulated dirt in places , but it didnt look like we could spot a leak.
anyway, the other day he told me that he removed the caliper from the bike frame, took out the pads and pulled the lever to see if the pistons were moving. Says one moved out, the other didnt much, but also said that he saw some leakage near the seat of the piston...
I dont have any hydraulic bicycle brake experience, but this guys experience is one that is exactly why I didnt want hydro discs. He doesnt ride in winter, he doesnt ride very much, and the bike is kept in his garage. Pretty average regular Joe bike experience, and the bike cant have many kilometres on it, is only 2 years old probably.
His mid level deore flat bar setup is a totally common and presumably reliable, and uses shimano mineral oil, so I dont know if hanging in his garage near the door it either got frozen and or maybe heated up a lot if it was near a hot water rad.....as it appears that there is a seal failure, fluid loss, and probably the fluid contaminated the pads too....
the other possibility is that he has kids and a pretty busy garage, so whol knows, they could have banged the bike with something, who knows, although the leak he says he saw at the piston doesnt bode well, and that couldnt be from an impact issue....and doesnt appear to be a housing damage issue...
so just a real life story of hydros and why I prefer mechanicals for never having to worry about stuff like this.