Originally Posted by TimothyH

Brake, not break.

Check that the brakes are bolted to the fork tightly.

Originally Posted by HTupolev

Check whether the rocking is actually happening in the connection between the brake arms and the posts on the fork. If it is, check to make sure that the bolts securing the brake arms on the posts aren't loose. If they weren't loose and the brake arm was rocking on the post anyway, then as long as it's subtle you're probably fine; cantilever and v-brake arms rocking slightly on their posts isn't unusual.