When I was converting a car to EFI I bought a pump to use in mocking up the mounting bracket and lines. I tested the pump and it didn't work at all. Afterwards I decided to take it apart to see what's inside.

After the cover crimps were peeled back I removed the cover and was surprised to see the brushes and armature were like new. The only thing wrong was that everything had a gray coating. That's why it didn't work.
 When I returned to the wrecking yard I realized that the truck it came from had been converted to propane, probably when new. The pump was left in place and over the years the coating had built up. This photo is after the coating was removed.

Now I wanted to get it back together and use it. The fixture shown was used to squeeze everything into the case. Only light pressure is needed as o-rings do the sealing not the crimp.
 The crimp was bent over by tapping it with a hammer.

The pump passed 2 liters of gasoline in 50 secs, and it's been working fine for months now.

Here is another one. It's surprising how small the actual pump part is. It is at left center.

The armature spins on the shaft, turning the 5 sided piece, which has 5 rollers in 5 grooves. The offcenter housing squeezes the gas and squirts it up around the armature and out the top.

The bottom has a pressure relief valve that recycles gas back into the entrance as needed.