The clunk noise will almost certainly be due to wear between the cradle (that goes round the disk) and where it locates in the main calliper body. As you probably brake whilst going forward most of the time the cradle is all the way forward so no clunk, its only when you brake in reverse that it shifts back and you get the noise. This explaination is probably a bit confusing, apply the handbrake, take the wheel off and rotate the disk back and forth and you will see the cradle moving. The reconditioned callipers I bought had the metal built up on the cradle location points which stopped the clunk.