I've started bleeding the new brakes (disc brakes from a 1974 Duster) on my 1965 Barracuda. Since the MC is dry, I'm bench-bleeding it on the car with some tubing that recirculates the fluid back into each reservoir.

I've gotten the front reservoir (for the calipers) bled so that no bubbles now appear but the rear reservoir (for the drums) doesn't flow any fluid what-so-ever.

Am I doing something wrong or is my master cylinder bad?

Try plugging or restricting the flow off the front port in order to help "prime" the rear section of the cylinder.
DD

An update.

I tried plugging the caliper port but that only caused fluid to spray past the plastic plug's threads. The MC sat dry for several months (years?) so I think the spool for the rear brakes was sticky. I had a spare new MC that had also sat around for several years and I gave that one a shot.

Luckily, I was able to bench bleed this master cylinder. I had a hard time getting the air out but eventually succeeded. If I get around to pulling the guts out of my first MC and cleaning it up, is there anything I can use to keep it in serviceable condition until I get around to using it?

Put fresh, clean fluid into the MC that you're going to store for a while, then cap all the connections and keep the cover on.

Remember that brake fluid likes to pull moisture right out of the air.... so fresh fluid and keeping it sealed will prevent rust.

- Mac

The problem with failure to bleed sometimes is that the supply opening, for allowing fluid from the reservoir into the pressure cylinder, is blocked, cruddy, or the piston travel is not such that it gets uncovered, so no new fluid drops in to replace the fluid pumped out under pressure. If the piston and the supply port don't align on each stroke to admit new fluid, not much you can do at home to fix that. Each stroke you should see bubbles coming into the reservoir as fluid replaces air.
This comes up on ATE systems from time to time, not sure if US systems have the same issue. With cheap repro parts, I'd expect it to be a worry.
Andrew

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1964 Barracuda Commando 273 (friend's)