Rob, my Excella also had the single handle by-pass set-up, and it also failed. Inside the metal box to which the handle and pipes are attached is a system of valves that carry out the function required. My enquiries indicated that spares were not available to renew the parts inside the box. I therefore junked it, and replaced it with the usual "two plastic diverter valves and short hose" system available in a package from RV stores for about $14. I would recommend that course of action to you.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."