My Landy (yes, it's got 4 wheels) brake master cylinder was leaking. No spare seals and very little spare fluid. I carefully dissasembed everything and cleaned it out with metho from the stove. The problem was the rubber boot had split, allowing sand and dirt to damage the seal. Once cleaned out, it held pressure with just a weep of fluid. The lack of a boot was solved by using a condom (I was single then). It tends to "melt" with the action of the fluid but will get you home.
I've heard several stories where to fix a buggered main or big end bearing, the shell is removed and a suitable piece of oil soaked leather belt inserted instead. If it's slighly too thick and needs tightening down really hard, so much the better. Drive carefully.
Tim ('91 R100GSPD)
The only baggage you carry should be in the panniers