I vote for the slave.
Water collects at the lowest point (the Slave), and rust seizes things up. It seems unlikely that the master would seize.
Also, (statistically) across the years with various cars, I have had far more slaves fail than masters, like 1 Master to 5-6 Slaves.
1) Open the bleeder. If the pedal moves and fluid squirts out, it's the slave.
2) If the pedal does not move, slave may be too clogged to bleed- disconnect the hose at the Slave.
3) If the pedal moves now, and fluid squirts out, it's the Slave.
4) If there is no dripping, or only a little dripping, but the pedal still does not move, hose may be clogged like the problem Jim had (very unusual case)- remove hose at Master.
5) If the pedal still does not move, I lose my bet, and I'm a humbled diagnostician.
Best of luck
1986 300E 5-Speed 240k mi.