Recently needed to rebuild rear master cylinder on my 2004 V11 Sport Ballabio.
Found the rebuild kits at Gutsibits in UK. Cost for the two kits was about 38 Euro.
The one kit is the piston, seals, spring, o-ring, sleeve, washer, etc. (BRA10241 / LS0418)
The other is the 90 degree plastic fitting and seal that attaches to body where brake fluid supply enters master cylinder. (BRA19500 / LP1419)
I suggest making sure to remove the 90 degree fitting, as that is where my problem was where the smaller of the two holes was plugged in behind the 90 degree fitting seal.
This resulted in the caliper not fully releasing, keeping the rear brake active, heating up the pads/rotor. Lucky I noticed fairly quickly ( the next time I went to apply the brake) as I think if one did not, it would quickly cook the pads/disc.
Delivery to Canada by post was relatively fast.
I rebuilt the rear caliper, new pads, as well using kits from Powerhouse UK.
Could not find Rear Brake Master Cylinder Thread so added here.
Picture is of the removed old parts.