The fan can be taken off easily:

Slide off a special spring steel locking clip.
Then slide the fan off the motor shaft: do not tap it with a hammer, because then the collector and the armature will slide on the shaft!
And then get the collector pushed back: I'm going to try that soon anyway.
Then I file or mill away the lugs, with which the housing is riveted to the front and back plate; do this on the back of the motor.
Then pull off the back plate with carbon brushes.
You can immediately see whether the brushes are short (=worn) or long (=good).
Tap the carrier for the fan out of the shaft.
Now you have the whole thing apart and can blow it off outside: lots of carbon dust!
If the brushes turn out to be good you can reassemble. Sequentially:
1- Push the carbon brushes 1 by 1 back into their holders and temporarily block them with a copper or iron wire which you insert from the side into the holder of the brush
2- Lubricate the slide bearing with a few drops of oil and slide the rotor into the back plate
3- Remove the 4 blocking pins, thus releasing the brushes.
4- Check that the magnets of the stator are clean, because iron particles like to stick to these permanent magnets! Clean if necessary with piece of adhesive tape or sticky compound.
5- Slide the housing over the rotor-brush combination. CAUTION: Make sure that the rotor is not (very) suddenly pulled out of the housing from the back plate by the magnets! Because then your carbon brushes will shoot next to the collector again. You can do this by inserting a crosshead screwdriver of, for example, 6mm through the bearing and fix the axis of the rotor in the back plate. Then slide the housing over the screwdriver, so to speak, so that the motor is closed.
6- If necessary, turn both parts until they fit together exactly.
7- Place 6 tack welds where the clamping was originally located
8- Replace the fan and install the clip. Ready for assembly.
I don't know if loose sets of brushes can be bought. You can unsolder the old ones though.

In the last picture you can see the permanent magnets placed in the housing against the inner wall and held in place with springs.

Written by my friend Kees