Thanks for letting us know the outcome, i had a similar issue with brushes, except my dynamo would run for a few hundred miles then stop charging, cleaning the commutator would give me another few hundred miles and so on, cured with new brushes. Its the first thing to check I reckon. In my case the dynamo would not "kick start" itself with f and d joined, but passed all other checks, it would start and keep generating if one put a little current through the field to kick things off. Too much resistance in the brushes was enough to prevent the "kickstart" was my conclusion.
I wonder how many people have bought new armatures etc when the problem was the brushes were faulty!