On the GM generators, the field current is created by residual magnetism in the pole shoes of the field coils, and a small voltage is induced in the armature. The field circuit begins at the insulated brush, and continues through the field coils to the "F" terminal. Try disassembling the generator again and taking the brushes out of it. There should be no continuity from the "F" terminal to the generator housing (checked with an ohmmeter) with the brushes removed. It sounds like you've got a field coil getting grounded to the generator frame that's letting the field current go to ground without being controlled by the regulator. That will put the generator in full output with no way to regulate it.
Jerry

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