I took my old Onan pump apart, Coil, sliding hollow piston, rubber seals for check valves each end of piston; mine had small white deposits on piston/ bore, (which I blamed on ethanol and I was blaming that for failure, as other defects not too obvious. However, in hindsight, maybe just old age, old rubber seals, unseen rubber seal damage/ tears? Similar (maybe higher pressure pump?) used for diesel fuel to feed to the INJECTOR PUMP?
(TerryH.) 2000-GS Conquest Limited 6266 Class-C 99-E450SD V10