That's a very odd problem you got. The things I would check is to make sure the pot and wiring are good. Does the forward and reverse contactors have interlock switches mounted on the bottom of them? One of the circuits in those switches is used to turn on the SCR controls, so check that. Does it have an optional fault detection card? But I think that would shut the contactor off if it detected a problem. Did you seat the new brushes to the commutator? I'll look at the schematic for that truck and see if there are any other ideas.