| You have taken the diaphragm out of the top of the master cylinder reservoir haven't you? The very first time I bled a motorcycle back brake, many years ago, I forgot to do this and had exactly the same symptoms you describe.
Other than that you might need to bleed the master cylinder first by just loosening the banjo a crack, pushing the brake lever down, nipping the banjo bolt up again, releasing the brake lever and repeating a few times. Press the lever SLOWLY otherwise you'll spray nasty brake fluid all over your paintwork and make sure there's lots of absorbent rag stuffed near the banjo to catch any drips - and lots of muc-off and water to hand if there are any brake fluid fountains going off. | 
Jools
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