Check the pistons out, I have the same problem and having removed the pistons from one caliper and checked them over, there is rust developing and causing the piston to stick, the thunk is the piston finally being released onto the disc. My other problem is that my discs are not true, one has a 1mm throw and the other 0.5mm throw, this is way over the maximum in the book. My plan will be either to replace the calipers or change the pistons to stainless steel ones and replace the discs.
Bob /ubbthreads/images/graemlins/cheers.gif /ubbthreads/images/graemlins/england.gif