Make sure that you have full length piston throw in the master cylinder. I don't know which master cylinder you are using or what pedal configuration, but custom jobs can lead to less than full piston travel caused by the sum of all of the modifications.
If you have full piston throw, try adjusting the rear shoes until you can't rotate the wheel. This reduces how far the wheel cylinder pistons need to move to make engagement, thus requiring less fluid coming from the master cylinder for bleeding purposes.
Back off the shoes to specs once you are done bleeding.
