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.
