My brakes do this first thing on damp or rainy mornings, no rust in the drums no apparent reason for this happening at all. All Components are new or less than 1 year old and not from Britpart, it's bled properly and adjusted properly.
I'm putting it down to the shoes on the front axle not being from the same batch and having a slightly different friction lining, they were bought from one of the suppliers in the Shiptonthorpe area and weren't boxed so may not be a pair.
I've a set of Mintex to fit this weekend so I'm praying to the gods of deceleration that this niggle will be cured... But I'm not holding my breath.