Originally Posted by RubeRad

My problem is now solved.

I went at the rim seam with some vise grips, and I was a little disappointed how easy it was to mush around. It seems that seam is not very strongly connected. Is that usually the case with extruded rims, or might this rim be defective?

Anyways, I got the seam to be a tiny bit 'downhill' (from the perspective of the brake pads), and now when braking I can feel a slight pulsing, but nothing like the strong thumping feedback I was getting before.

What I learned from this experience is that the rim is necessarily a dynamic system under braking force, and I can't expect it to be all that rigid. So I'm happy I got it as good as I did, and plan to not fret about it in the future.