They are a pain  to bleed.If you have a 1 man bleeder or any type of suction device(even put a hose on the bleeder,and suck on it to get vacuum)just enough to get fluid through the line.Get vacuum in the hose then crack open bleeder.Repeat as necesarry.

Here's an old-school way to bleed the master cylinder:

Grab you an absorbent rag you don't care about.

Fill the master cylinder reservoir with brake fluid.

Look at the inboard side of the master cylinder assy.  You'll see a banjo fitting.  Pump the brake lever a few times and then keep it held in all the way.  With the rag wrapped around the banjo fitting, loosen the bolt on the fitting a bit.  You should see some brake fluid spurting out around the banjo fitting.  Holding the front brake lever fully closed, re-tighten the bolt.  Slowly release the lever.

Repeat until you don't see any more bubbles coming out around the banjo fitting when you have the lever squeezed in.

You may need an assistant to help you perform this procedure.

DON'T let the reservoir go too low on brake fluid.  DON'T let any brake fluid drip onto the bike.  If it does, clean it quickly with soap and water.

IHTH!

Apparently, when the master cylinder( m/c) gets emptied & the fluid gone from just below the reservoir, it doesnt want to prime back up & pump. The suggestions have offered ways to drive fluid back into that area, so fluid will be pushed when the lever is squeezed. It might also be possible to remove the hose from the calliper & hold it high & fillit, using a syringe. It wouldnt have to be Full, just enough to put fluid under the M/C. A good stiff wire & some tape would allow you to mess with it & not need to ask the wife to come out & help. I would think that just the slightest pressure on the fluid in the M/C would be enough. Maybe a rubber glove taped on the M/C & blown up some?  Lots of ways to skin this cat.