Deejay, That sludgy build up is what spurred me into action re. a proper flush rather than just a coolant change. Again my old fluid looked fairly new when drained yet the flushing process proved that clay like greasy brown debris was present throughout the engine and radiator. I've heard of the sterident cleaning method on header tanks but as you rightly described, the Ford header has a complex compartmentalised design and I felt to get the Sterident fully throughout the whole tank wasn't really feasible. Instead I used a washing machine liquid detergent in hot water using a bucket. I was then able to manipulate the header tank under the water to fill all the compartments. Left overnight it did a fairly good job. there is some slight discolouration to the plastic due to hot coolant but when refilled with the new coolant you cannot see this.
