Xantia II 1.9 TD '99, few questions.

[chinkostu]

Sounds sensible enough. I found that there was quite a strong flow from the bleed point by the fuel primer, so I had a clear tube attached to that running back to the header tank for a few minutes (until there were no more traces of air visible when massaging the rad top hose or revving the engine). Coolant level dropped quite a bit first time out, but still was water in the tank at least.

What is the square electronic thing on the header tank for that matter? Would say level sensor normally but there isn't a warning light for it, so seems a bit pointless if that's the case...its connected, so not a replacement from a different model or anything...

It is infact a level sensor!

[falling-out-with-my-car]

The black sludge could be what used to be called a dog turd, not a real one but a chemical shaped like one
crumbled into the cooling system to fix leaks, they create a residue which builds up in the bottom of the expansion tank.
after the system clean if you do have any leaks try K-Seal its £12 a bottle but hey what a brilliant sealing product and it mixes with all types of coolant.