Valleys tend to become rusted due to the amount of water they collect and the moisture that is placed on them by the water retention of the mortar sitting on them when valleys deteriorate and tend to leaving home owners wondering where the leak may be coming from. Valleys that show superficial signs of rust are probably having further damage that may be hidden by the mortar on each side of the valley so we advise that removing and replacing your old rusted weathered valleys with new Colorbond or Zincalume ones and replacing the old mortar with contemporary storm seal should get rid of any danger of them and causing interior damage.