If you are picking the Brake to run the CPU thinks everything is OK, therefore whatever tells the Generator Field to Ramp up is not working, or some sort of ground in the Brush Rigging that only shows up on Inspection. If you have Motor Field and the Motor Armature is good, the only other place to look is the Generator Field and related circuits on Inspection.
Yep, so if your IGF is the same voltage but the polarity is reversed (profile pattern up and down) then the profile or ramp would indicate its ok, then there may be a small generator leak somewhere. If it isn't then the either the processor isn't pumping out the right profile, or something else is stopping it from doing it.