(Embarrassing question. I had a vague sense of how the master cylinder (M/C) works; but, when I look at the pictures, I don't see it.)
When bleeding (per schematic below):
Push on the pedal. The piston moves right, and closes off the intake port. Open the wheel cylinder bleeder. The M/C's piston cavity (where the spring is, in the picture) is compressed, forcing fluid out through the bleeder. Close the bleeder. Release the pedal.
The piston wants to retract to the left. The piston cavity needs to expand. For that to happen, new fluid needs to be supplied from the reservoir to that cavity. But, isn't the intake port closed off at this point?
Q: How does the bled-out fluid get replaced in the M/C piston cavity?