I have to confess this was one of those jobs I had been putting off, who wouldn’t. However it turned out to be straight forward and not unpleasant at all. Having “Googled” the fault I was reasonably sure that the culprit of the back-flow was the poorly named joker valve. The source of other the leak could be one of many. I opted to buy a complete new pump assembly figuring that I could take my time to service the original and would then have a complete spare in the event of a major failing on a long trip.
For obvious reasons I flushed plenty of water through the system before taking it apart and of course pumped it dry. The seacock for the inlet water was closed before simply loosening the jubilee clips and removing the hoses. I first unbolted the flange that holds the joker valve and the problem was immediately obvious. A stone like material was blocking the joker valve partial open. The pump assembly itself was simply removed after removing the retaining bolts. Before installing the new pump I checked the outlet hose leading to the holding for any signs of the stone like material. Surprising it was completely clear so not further action needed. The new pump installation was simply a case of plug and play, reversing the dissembly procedure. And that’s it, a new properly function pump.
Footnote
Obviously the stone like material is not stone.
Before
A Dragon's Tale 