Finally said fuggit and did an eBay buy for a whole brake system for $125. Hell, the secondary master cylinder alone new was over $200.
I bolted in the new left caliper & secondary master cylinder assembly, attached all the banjo fittings, hit it with the mity-vac bleeder, and my back tire locks no more! FINALLY!
Just need to finish some bodywork and I can get this dude on the street.