Replaced F&R stock brake lines with some braided stainless. Have done this several times before on various bikes. Fronts are fine, but can't get pressure to build from/ in the rear MC. Its an MC21 unit, and previously worked fine. Tried pulling fluid through from the bleeder end using tubing and a large syringe. Also tried removing the banjo bolt on the MC and trying to "prime" it with some fluid, but there was already fluid in there. This is the last hurdle before I try to restart the beast. Any help appreciated. I have an MC28 rear MC sitting here, but the attachment holes are slightly different.
Thanks
Jeff, Ohio

Mine was a bugger to bleed as well, and I thought it might/may have just been shagged. In the end I took it off the bike and did it on a bench with a spare disc, and keeping the MC higher then the brake cylinder. Then slotted it all back on. Was a pain in the butt but seems to have worked.

My own preferred method is to use a very long piece of tubing,run it from the bleed nipple over the seat unit and back down the other side into a container (old oil can). That way you can open the bleed nipple and leave it that way for any bubbles to rise, since the fluid will sink and stop air going back in.

You can leave it like this overnight and usually the system will have 'self bled' by the morning...
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

I had the same problem and ended up using a homemade brake bleeding pump

I had same problem with new pads. It looked like the piston was pushed so far back into the housing it wouldnt allow fluid past it. Did as someone else suggested, took it off the bike, removed the pads and put a piece of thinner rubber in there, bled the system, the replaced the pads.

the MC won't build pressure. Tested it against the MC28 MC sitting on the bench, and the 28 is fine.
Are the guts of each the same, i.e. are they interchangeable?
Whats involved with rebuilding one of these? Looks pretty straight-forward, but the rubber boot on the bottom edge looks like a fragile part.
And finallllly, where can I get a rebuild kit for the stock MC21 rear MC?
Thanks all
Jeff

Decided that I had nothing to lose, so disassembled the mc28 master cylinder, swapped the guts into the mc21 master cylinder, and it works great. The 21 MC had worked the last time I rode the bike, about 3-4 years ago, but internals had frozen-up and dried out in spots since then.
First time I've torn into a master cylinder, and glad I did.
Thanks to all for their help, advice, and links.
Jeff