Grob 103 Twin II Brake Master Cylinder

Everyone has covered the topic well but I'll add my 2 cents having
repaired our 103 brake. Yes, it is a Magura master cylinder as used
on BMWs. All they did was cut off most of the brake lever and mount a
cable to the stub to actuate it. Yes, bleeding from the bottom of the
system (slave) works best. Here's a little trick - any marine supply
depot sells a plastic pump for replacing bottom end oil in outboard
motors. It looks like a shampoo or epoxy pump and screws onto a
standard oil, or coincidentaly, brake fluid bottle. Screw it on the
bottle, push the tubing onto the cracked open caliper bleed nipple and
pump slowly. It forces the brake fluid up. Have someone at the
master cylinder with a towel wrapped around it to take care of
overflow. It is always easier to force air bubbles upwards in a
hydraulic system then to force them downwards. It is faster then from
the top and a whole lot let frustrating.

Another hint ... don't worry about replacing the entire master
cylinder, Just go into a BMW motorcycle dealer and buy a rebuild kit
(about $75). It includes a new piston, spring and seals. (I can't
recall the bore size but these kits care reasonably universal - all
you need to do it confirm the size at the parts desk.) Use a very
fine emery cloth or wet sandpaper to gentle hone the cylinder and put
it all together. Usually the problem is the seals, not pitting of the
cylinder. Total job takes about 1.5 hours including removing and
replacing the cylinder.