I just put my CR-18's on my bike and went for a ride. Every time I actuated the front brake, there was this "bump" once per rotation. What could this be caused by?

Find the bump, describe the wheel in that spot... Could be out of true, could be a rock in the brake caliper, could be something in the tire, could be a loose spoke or could be, you ran over a unicorn

My guess is that the seam isn't flush where the rim is joined together. But just find the spot where it's grabbing. That will tell you more than we can online.

wheel out of true (side to side).

Wheel has a lump in it (for example from a hard impact that blew out a tire and bent the rim).

wheel has a spot of grease on it

tire is not seated on bead, or has a bubble, or is somehow rubbing the brake pads

brake pads are not centered, aligned, or wheel is somehow losing contact with the pads for some other reason.

you should be able to find the cause visually. Spin the wheel, and gradually apply the brake.

well theres your problem bro

It's definitely the seam. I noticed, when I got them, that the seems were very visible, which was a bit confusing for me.

What course of action can I take to correct this problem? I ordered them from Velo-Mine over Christmas, and this is the first time they've been on the road.

If it really bothers you, you could contact them and try returning them. IMO it's really not a big deal. Run the wheel "backwards" if you can, so the brake pad isn't jamming the seam everytime. Or carefully file it down flush. It may never be perfect, but you can make a big improvement with a file and something to smooth out the filing.

EDIT: I guess what I meant to say is that it's not a big deal once you fix it the best you can.

It is a noticeable jolt that rattles my handlebars a bit, so I consider it to be a big deal. I had thought about flipping the wheel around to see if that helped any. I contacted them to see what they thought about the situation. I'll be going out for a ride later tonight. I'll try flipping it around to see if it helps any.

Sand it flush with really fine sandpaper. The sidewalls of CR-18s are machined anyway so if you're careful, you won't even be able to tell when looking at it.

Its not the brake, your headset is loose.

no logic can support that. It is the seam of the rim bro.

Legit. I've seen countless rim seams that do this.

Try flipping the wheel around. The seam may bump worse in one direction than the other.

I missed the once per rotation thing lol, when my headset was loose it would clunk when I used the front brake.