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I've recently got new deore brakes which have been great, except on the last couple of rides they seem to go solid at the bottom of steep descents. Literally feels like I can barely move the levers. The brakes still bite, but with almost no pressure on from me. After a bit this seems to ease and they are fine the rest of the time.
Can anyone help? M I over braking? Is it the brakes overheating? Do I just need better brakes or do they just need a bleed?
That's the brakes "pumping up" due to the heat. Air in the system certainly makes this worse so worth a bleed. However, older Deores used to struggle with heat more than the posher models, not sure if the same is true of the current crop but worth bearing in mind.
How long are the descents? Are you covering the brakes all the way down, or do you only apply brake before corners and any other stuff?
Air in the system will not help as said ^. What compound pads are you using? Maybe the finned pads would help as IIRC, the 985/7's will fit into the caliper, or that maybe down to SLX and not Deore.
Could this be the reason SLX and XT are fitted with ceramic pistons?
I've got some old 525s and they don't like getting too hot, they don't pump up like yours, the power just fades away to nothing which makes me think its a caliper/piston heat managment issue, I assume there must be a reason for Shimano fitting ceramic pistons to other models.
Would try a bleed first.