The Stovebolt Site Forums The Shop Area Driveline '51 3100 brakes: is it a leak, or does brake fluid evaporate?

My first guess would be line rusted through...then a wheel cylinder leaking.
If no signs along the brake lines then I'd pull the wheels/drum off and inspect. If its been very long the rust, old grease or brake dust, mud, and crud may have soaked up the fluid so you may have to pour some in the M/C and pump them, maybe even bleed them to find the source.

....and yes, definitely get it to Whoa before you Go again. Lets us know what you find.