The things I learned dealing with my crapola problems:
1. You can get deadspots in pump blocks if you use the 'wrong' inputs. This wasn't something i knew of with second hand parts in my very first attempt, as I was missing pieces and mixing brands.
This is what caused the mystic fog to clump up, the particles would accumulate since they weren't being stirred up (And once clumped, it never separated again)
You can see how things could accumulate down here easily if this wasn't leading directly to the pump to be drained away - but that fitting on the right if unused because you returned the fluid in via the top of the reservoir cap, is a spot where debris can accumulate and build up on you since it's intended to have fluid constantly moving over it and now it's a little dead spot for things to get stuck
2. Other components can do the exact same thing, as I learned with the cockroach. That hid in there for a YEAR.
I needed to blast it with water straight from the bath faucet to get that thing out (I'll use the garden tap less time so it doesnt blast me in the face) - flushing may not get the job done by replacing the fluid alone and letting it accumulate, you may need to tear everything down and (carefully!) go to town on the radiators with higher water pressure. Small debris could clog a radiator single fin and linger for a long time.
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