1990 searay 310 sundancer duck Bill valves

[Paul bfield]

It seems I may be getting back flow into the head. Originally the water would always turn black in the pan then after cleaning and sterilising the holding tank we had some blue water in the pan. I hear a lot of talk saying you must change the duck Bill valves but I can't for the life of me workout the location of these (does this model have them)
Pics would really help please
Update I found the valves but it seems the previous owner may have done a few bodge jobs, there's actually a 5 gallon fuel tank been converted to act as something in line. Does anyone have a correct diagram of how the head plumbing should be run

 

[FINAL DRAW]

On our 1991, same boat the pomp is in front and separate from the holding tank on port side in engine room fastened to floor. Duct bills are in inlet and outlet hose fittings on pump. Note direction for duct bills.

 

[Paul bfield]

Thanks we think we found them but I believe someone at some point has tried to bypass this as from what I see the head goes directly into the tank