The booster could have a stuck valve or piston or a torn diaphragm, none of which would show on the outside or cause a leak. The master could be stuck.
If the fluid came out clean from the start of the bleeding process, the hydraulics could be OK. My guess is it was rusty brown and all the cylinders at the wheels, master and booster are loaded with crud. I got lucky with mine, the master and booster had been replaced shortly before my purchase. All the wheel cylinders were toasted, only one leaked externally.
