they are (can be) an absolute BU55ER to bleed. Do them one at a time, starting with the one furthest from the lever/master cylinder (this is usually the left hand caliper unless you have 'front ended' your bike!)
I use a little one way valve device for bleeding brakes, can be got form halfords (or i see them on Ebay regularly)
with this on the nipple release the nipple, and remove the cap on the master cylinder, (ensuring you have a can of dot3 fluid to hand) start working the lever, eventually (10 mins to several days) you will see fluid appear in the clear plastic tube of the bleed device (make sure the master cyl is kept topped up) keep working the lever until there are no bubbles coming through, then close the nipple and repeat on right hand side. If after all that there still appears to be little or no pressure you will need to go through the proceedure again (and possibly again)
One point worth mentioning, make sure the hole through the nipple is clear (you will need to remove it, and poke it with a bit of locking wire or such like. I spent half an hour poncing about before i worked out it might be blocked. AND, if these nipples havn't been worked on for some time they can be siezed in, so be careful and gentle when undoing them, i've sheared one off in the past, which renders the caliper useless.
My Son uses a method that I havn't tried, that of pulling the lever back to the bar, and tying it there, closing the nipple, putting the top back on the master cylinder (to stop damp getting to the fluid) and leaving it for 24 hours, air should rise to the top. thats the theory, I have my doubts
