I was being harsh, but I've put all sorts of things into cooling systems, central heating descaler works especially well on ally heads, but in the end if its running to temp and the heater works its doing fine. Yes sure there are some solids, but that's no different to a central heating system and by and large every time you drain and re-fill you just put in more oxgenated water and kick start the corrosion all over again. Most everything you can add to a cooling system is either searching so it will leak or attacks something that also causes leaks. I would file it under "character" and leave well alone.

kermit_rr said:

Don't forget the radiator and cab heater, if you have one, can be full of gunk

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The cab heater pipes seem to both feed into the heater matrix from the top.

Is it best to disconnect and unbolt the matrix and flush it out of the vehicle. I fitted a used but good condition matrix when I rebuilt the vehicle. I flushed it with a hose before fitting it, so would hope that it hasn't filled up with too much dirt since 2015.