[T Creighton]

Hi Daryl,
You have overcome many problems along the way with your clever lines of thinking so I hesitate to offer this theory but it may be helpful (or not).

When the brake shoes are adjusted out to the drum, the cylinder cups will follow the shoe due to the spring between the two cups.
In a vertically mounted cylinder this could result in a cavity above the inlet port and below the uppermost cup that air could be trapped in from when the lines were first installed.
Being above the inlet port the air could remain trapped when bleeding the brakes.
If you were to slacken the shoe adjusters right back the shoe springs would draw the shoes back and move the cup down closer to the inlet to reduce the cavity and the risk of trapped air.
Then bleed the brakes.
Then re adjust the shoes out hard while holding some pressure on the pedal at the same time.
Then release the pedal and re adjust the shoes for clearance.
Best Regards, Terry.