Looking straight down in the m/c you should see a small hole. Probably about 1/16". Depending on the position of the piston inside the m/c, when the piston returns, it may not be letting this port do its job. There is a procedure to check this but can't state it off the top of my head. I had this same problem with a 47 Dodge car. The actual problem with it was the m/c was new and the port hole was not drilled all the way through.
.......sometimes a known procedure just don't seem to work. I ended up putting a hose from the bleeder valve into a jar with some brake fluid in it to cover the end of the hose and just pumped until no air bubbles. This is just yet another idea.
Keep us posted. We all want to know what you find.
