Dirty Fans, worn brushes
All add up....think of it as a snowball effect. If the fans were stopped up and filthy, it's not going to be running at it's full potential, simply because it can't breathe....you know when you have a cold and you're all stopped up how sluggish and short of breath you are....same concept (well close enough anyway!). Add in the worn brushes, that can cause the motor to slow down as well, now you REALLY have a problem on your hands!
Just out of curiosity, how do you plan to seat those brushes when you get them in? Some folks use the VARIAC method (where you cut the voltage down to 50-55 volts and run it at 1/2 an hour), or if you're REALLY feeling brave, taking a seating stone and stick it to the commutator while the sucker is running (no pun intended!)....just wondering?