On my first extended ride of a new bike (see: http://www.khsbicycles.com/08_urban_xp_06.htm), I noticed a distinct skip on every revolution of the front wheel when applying the brakes (linear pull). At first I thought the wheel was loose. I reinstalled the wheel, lifted the front end and pounded down on the top of it to make sure it was tight, but when I rode again, the skip was still there. The wheel appears basically true and I can't see any imperfections on the rim at the point where the skip occurs. The bike shop mechanic tells me that this is natural and caused by the rim's seam, and that after a while enough brake material will lodge there to make it unnoticable. I'd never heard of this. Is it common?