Hydraulic brake problems

Hydraulic brake problems. The disc brake on my D2b has provided only
very limited stopping ability for a year or so now. When attempting
to put on the brakes the brake handle moves all the way back to the
stick grip. We have repeatedly attempted to eliminate air in the
system by pumping .4 fluid up through the wheel cylinder drain
nipple. We have sucked the system dry by vacuum applied, also to the
same nipple. We have checked the nipple itself for blockage. We
replaced the master cylinder. We checked the cable to make sure there
was no slack yet the abbreviated handle plunger was virtually in
contact with the piston in the master cylinder and the master cylinder
piston was moving. Nothing seems to make any difference. IT