This tip came from a radiator repairer who told us one of the most frequent causes of radiator tube blockage is the excessive use of silicone rubber for water pump and thermostat gaskets. The overflow or excess to the inside cannot be cleaned and eventually some of this will break free with the water flow and carry on into the radiator, in many cases lodging in the tubes and blocking them. Silicone rubber will not dislodge or dissolve once in the radiator and dismantling it is the only way of removing the silicone rubber from a blocked tube. Remedy, use proper gaskets with a good recommended gasket cement. The same can also apply where there is oil under pressure. Again use recommended gaskets and go easy on the silicone rubber in a tube.