Had the same troubles last summer. Turned out the bleeder valve was bad

 

I had to pump my master cylynder about 200 times before it finally started pushing fluid down into the line

 

Have stroked rear pedal over 1,200 times 4 different days. Most strokes get tiny bubbles coming out the speedbleeder.
XL883C 2005 Replaced rear master cylinder, added new DOT 5 and a SpeedBleeder.
Lots of tiny bubbles on most strokes, a few bubbles 1/8th inch. Varied strokes, mostly full stroke to bottom, slow, fast, partial slow strokes. No leaks. Bike vertical.
Speedbleeder.I've never had brakes bleed like this. I was pumping so much new fluid that I routed the bleed tube to the reservoir. (I bled the front brake fluid no problem.) No Joy. What to do?
Kayakeur Bud May 26-2015

 

the master cylinder has to be perfectly level to bleed.

 

Bleeding: Why is it not good to push the pedal all the way down?