This is the first I've seen with the pump on top. Usually, it's a pump assembly that bolts right onto the face of the motor. Have you contacted National about it? I'm not completely sure what I'm looking at here... never mind, I see National is gone and parts can be had from Global somebody...
For the slight suction you are getting, and the motor is spinning normally, I feel the pump is the culprit. It would be nice to have a reference point, such as "I've felt it before, and it's not as strong now..." Can you blow air through the hose to be sure it's not obstructed, and lastly, does the motor sound like it's suddenly working overtime due to drag from the pump? Can you hook it up to another vacuum to test it? That's all I got...
Steve
I just saw the post about the carbon vanes, my pump has those. I've occasionally had to take the pump apart, not very hard at all, and clean and oil lightly the vanes and reassemble. If the vanes are stuck, then they won't slide out and pull any air. If you get it apart and see it, you'll understand how it works, very clever.