Generator rotation

[Gunfreak25]

This question is for the 24V, 25 AMP generators used on the M37 and other early M series vehicles. What would cause the generator to only make power in one direction, and zero power in the other? Mine puts out 36 volts (measured without regulator, duh) running counter clockwise as viewed from the drivers seat. I am putting a different motor in the truck that will turn the generator clockwise, which means no juice! Is it just a matter of repolarizing it? Do the brushes get used to running in one direction or something? Broken?

I have the mechanical inclination to rebuild it if need be. Just hoping somebody could explain this for me as I know little about generators/alternators.