I'm not familiar with 320 brakes, but on many modern cars (that I consider the 320 a modern car betrays my age....) with single piston calipers, the slide mechanism needs lubrication, especially if it is somewhat worn and allows the caliper to cock sideways a little.
On my Miata, I need to lube the sliders (esp. the rears) yearly, as a maintainance item, or they'll stick.