The problem with vac motors can be the bearings. When the bearings start to wear out, the comutator (the bit that spins round) has more free movement, allowing it to run out of line putting a strain on the motor and causing irreguar brush wear, or at worst to hit the armature which will knacker it completely.
In theory, if you look after the bearings, keep moisture / water away from the motor, the only other thing to do would be to change the brushes. If you look at the area where the brushes make contact, you will notice that around the shaft there are grooves, and these get choked up with carbon from the brushes, which will eventually stop the motor workig, but not in the short term.
When i worked in the cable industry, if one of the extruder motors failed, it would be stripped off the machine by the fitters and sent away overnight to be rebuilt and have new copper windings, at a cost of a £1000 or more.
If you are ever stuck, i think i can get you their number :
:
Dave