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Can I use I-Spec B shifters with conventional clamp brakes?
What brakes? If its anything shimano since about 6-7 years ago, yes (with the right adaptors), other brands generally no although some convertors do exist.
You may need to buy a new shifter cover like:
http://www.sjscycles.co.uk/shimano-xtr-sl-m9000-cover-right-y01u98010-prod37252/
To make it work.
Brakes are XT, probably 4 years old, so might need to look for adaptors.
That shifter cover costs more than I normally pay for shifters!
A new boxed iSpec B shifter should have the right adaptor - silver barrel looking thingy with a square head. That will convert your (almost certainly) iSpec A levers to I-Spec B. Are your XTs the M785 ones (ie the ones that were current 'styling' until a few months ago)?
Brakes are older than I thought, M775 bought in 2011