Since they obviously don't clean their blocks out after soldering them, there's a good chance that there is also some flux residue in there. Water alone may not remove those types of compounds. Consider filling it up with some kind of strong solvent and letting it sit a few days. Warming it and or vibrating it will accellerate the cleaning action. I would not pump the solvent around the loop, since there's no way of telling how it will effect the pump and plastic tubing. After soaking it, flusk it in the sink with good and hot water for several minutes. Poke around in there with some pipe cleaners or a test tube brush to knock off solder beads and other debris adhered to the walls and joints.