Hi Brian,
when we clean out our fire pumps at work which need to be flushed through after being activated we use a tank of fresh water mixed with a small percentage of white vinegar, this has turned out to be the most productive method,
we have experimented with many shop bought products including industrial descalers and dilute cleaning solutions, but we found this to cause problems with any parts made from aluminium and especially the impellers, we also found that any residue left over in the system would harden and cause even more problems,
so in a nutshell keep it simple as you would do with servicing the engine or cleaning any other part of an engine, don't use anything that will be aggressive to the hidden delicate parts
hope this helps
regards,
sam