The valve to the overflow turns into a hole in the side of the radiator opening, which for some silly reason makes a 90 degree turn within the metal of the radiator opening. So cleaning it is hard. A drill bit will unblock the external part going into the raidator opening, but nothing will get around the 90 degree angle, as it’s inside the radiator opening and can only really be accessed with a piece of long wire. Which has so far been unsucessful at unblocking it.