I recently had an interesting time with my header tank. After a split seal which caused complete loss of water and an overheated engine, I topped up the header tank again with water and AF. A week later, I noticed the engine was creeping over its usual 80 degrees temperature.
Another top up of the header tank and all was well.
A week later, and the engine crept over the 80 degrees again to nearly 100.
Topped up the header tank again.
This happened 3 times in total and so I thought naturally I'd got a leak. However, the fourth time I went to fill it up, it took very little water to top it up! Ever since, it's been fine requiring no further top ups....I can only assume that the earlier 'fillings' weren't in fact 'fillings', even though it bubbled over the top of the filler cap...
I concluded that there must have been an airlock somewhere and only by these repeated toppings up is it now, actually full!