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Can anyone tell me if there should be a small 'o' ring at the spanner end of a bleed nipple where it enters the caliper body?
Specifically XT M765 pm ones?
Yes according to Shimano Techdocs.
Time to rob my old M555 caliper as they have the same 'o' ring.
Not on mine, and it doesn't do anything anyway. It's there on others and AFAIK it's just to keep grit out of the thread.
I thought so too until today. I have a suspicion, when bleeding from the caliper up to the lever it stops air being drawn into the caliper through the nipple threads.
it's there (or should be) to stop oil/air moving past the thread.
so, i'm a geek, and used to work as a hydraulic designer, the first time i bleed a shimano brake i take the nipple out, and wrap it in ptfe tape.
threads are not air/oil tight.
If you're pressurising the syringe then air can't get drawn in anyway. If the bleed nipple is open too far then pumping the lever could overcome the surface tension and suck air in on return, but I don't think the o-ring would stop it in that case.
Try bleeding from the top while cracking the bleed nipple just enough so that a hard squeeze on the lever forces fluid through, then close it before releasing the lever.
EDIT - I like the PTFE tape idea.. Will try that next time!
Thats what I did. Might try raising the caliper above the lever and pump fluid up from the lever to the caliper. should help any air rise to the top too.
I found the problem, nothing to do with the bleed nipple and everything to do with the piston seals letting air past them FFS.
sounds like a leaky seal - you can buy a complete brand new calliper with new pads for £25.
either fit it and be happy. or fit it, strip down the old calliper for spares and be very happy.
Done and already done.
Deore 595 caliper ordered last night on CRC (same pads y'see)
Old caliper robbed for bleed nipple and M555 caliper installed temp until new caliper arrives.