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it is routinely stored upright
It can do sometimes, air from the top of the reservoir might make it's way into the line if it's upright, usually pumping the lever a lot will sort it, if not try the burping trick I mentioned, if you've not got the kit then just just undo the lever bleed port and try pumping the lever a little carefully and tapping the hose, you can drop a few drops at a time of fluid in if you can't see any in there. You can stop it happening again by whacking some straps or similar round the pulled levers when storing upright, although usually a good bleed is enough to avoid needing to do that until the pads get pretty worn.
snottyotter-
Thanks again for the help.
I managed to burp/lever bleed yesterday and got lots of air out (20+ small bubbles) and topped up the fluid as well. Feels much better.
On closer inspection I think oil has been leaking from where the hydraulic line enters the caliper. I've nipped up that connection so hopefully that might solve it ? Is there a good way to monitor that?
ILb
Thanks for the replies, it is routinely stored upright (hanging from the front wheel) - is this likely to cause issues?
It was making a fair old racket but a simple hose down with water seems to have relieved that, so I don't think there is a leak but it might need closer inspection. Some power returned with the noise gone but lever feel is still horrible.