Driving the Jeep out of the shop, on/off the trailer, and into my garage revealed it had almost no braking capability. The pedal went straight to the floor and only the last 1/4" or so had any braking effect (maybe pushing air at that point?).
We tried to add fluid to the master cylinder/reservoir in the floor but the cap was jammed shut. When I tapped on it with a hammer and punch it broke the plastic tab on top. The internals are all probably rusted anyway so time for a new master cylinder.
There's no shortage of original type and upgrades. CMVJeeps on eBay had the best description and actually pointed out that late-1948 CJ2A has a slightly different master. The mounting holes are slightly different and using the wrong style requires modification or different hardware. Sure enough mine is the later style so this seller saved me a bunch of hassle.
Removing was not too bad thanks to decades of oil and grease on the hardware. What really frustrated me was the old rusty cotter pins. They snapped away with almost no effort leaving a tiny little remnant on the shaft. And there was no way poking the pin through its hole was going to work. I ended up poking, prying, and filing the nubs away until I could get things to come off. I must invest in some stainless pins for reassembly.