Problem solved! The root cause of this particular problem is that---I'M AN IDIOT! In the end, I disassembled the MC that I had just put a kit into days ago, and I had assembled it such that the primary cup was blocking the "exit hole". In other words, I had the primary cup on the wrong end of the spring. It should've been up against the piston, and I had it down by the valve. That explains why fluid was coming out of the vent hole in the filler cap--it had nowhere else to go!!

Lesson to be learned by all: if you have a shop manual with step-by-step instructions and exploded diagrams, use it! I reassembled this thing from memory because it only has about 6 pieces, but I screwed up.

Once I changed things up, I bench-primed the cylinder as some of you advised. Now, I won't mention that once I started bleeding the line, I had an additional bleeder valve open and pumped fluid on my shop floor. That would be too embarrassing to admit.

Thanks for all the help. Live and learn.

DJ
'51 Chevy 3104 1/2-ton 216