One thing I just thought of. If the drill doesn't run at all it could be that the brushes are the problem if the commutator just had a spot of epoxy it would still work but have a dead spot in the rotation. Under no load the inertia would keep it spinning. Before opening it up again spin the chuck and see if it can run intermittently. If not the brush probably need to be cleaned. 20 volts won't jump a gap.