Hi, I guess you will have decided what to do by now.
Having just spent a lot of time on&off trying to get to the suspect motor on my Powa 4000 (and currently being stuck in trying to get the grey plastic cylindrical cover off the top of the motor so I can view the commutator and brushes) I'm not sure whether it was worth it.
Still I'm hoping I CAN GET AN ANSWER to the cover problem eventually on a forum (phoning spare & repair shops didn't produce any useful help).
Anyway to take it apart I used an exploded parts diagram that I found at
http://www.stokesap.com.au/downloads/Vax_Technical_Draw_LR.pdf
This pdf includes a lot of older models. The Powa 4000 sketch is hard to read and doesn't show much detail (but it was enough). If you still want to attempt it I can post bare bones notes of how to get to the motor. I've also sketched a simplified circuit diagram. Make sure you are getting 230V through the rectangular On/off pushbutton before you go too far as I think might fail by 15 yro! I found a lot of carbon brush dust in clumps of fibres stuck in gaps and crevices around the motor which I've pulled away but probably more of these and worn brushes and perhaps motor winding problem still lurk beyond reach.
BTW Vax Service Centre currently have the similar 6131T model at a special price of £99 or you can your vac collected, repaired and shipped back for £60 They will give you a new 6131T if the can't repair it.
Cheers,
Andy3D, February 2011