I just got done removing and disassembling a master cylinder in my '49 3100. My truck was suffering the same symptoms as yours.
Decent feeling pedal.
Nearly no braking action.
Brakes lock up and won't release right away.
Here is what I found inside of the master cylinder.
Varnish plugging the tiny compensating port hole which goes between the fluid reservoir and the main cylinder. I can shine a light from inside of the main cylinder and see it shining through the varnish. I sprayed some Gun scrubber solvent in the in the reservoir and it refuses to go through that hole.
As soon as I can find my welding tip cleaner tool I will open up that blockage and reassemble and see how it goes.
This is a very low mileage master cylinder. Seems like you get a 50-50 chance to get a new one which works.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission