You may be running the rear reservoir dry. I do not know which model you are working on. Sometimes the fluid level of the rear reservoir is very hard to see and there is a pin hole near the top connecting the front and the rear reservoirs, no other connections. Try to fill up the front reservoir all the way to the top. Wait for the brake fluid get into the rear reservoir and then refill again until the rear is full before bleeding. Use a flash light if necessary to check brake fluid in the rear reservoir. Be sure the pin hole to the rear reservoir is not clogged.

I hope that helps. Good Luck.

David