It sounds like you have an open circuit, not a short, on some of the armature windings. Current enters the armature through one of the brushes, passes through the windings, and finds a ground at another brush. If the conductors are broken, or the solder joint at the commutator goes bad, the current can't flow when the armature happens to stop with the damaged connection under a brush. Generally, an armature is not replaced during a "rebuild"- - -the commutator is machined smooth, and it's checked for open circuits, shorts, and grounds, and it is only replaced if it fails one of those tests.
Jerry
