Mark Etheridge at Moto Guzzi Classics just did a video on that issue…neat trick
Take the master cylinder off the frame, then tip it forward and pump the pedal. Then tip it rearward and do the same until bubbles come out
Repeat forward and back again…
The air gets trapped inside the smallest portions of the master cylinder. If you’re on the side of the bike looking at the MC, you want to rmove from the frame and rotate it clockwise and pump a few, then rotate all the way counter clockwise and pump. Repeating until you get a good shot of fluid lifting out of the MC with each stroke
Mystery solved! Check out his recent video