[C320-list] Head Back Flow

[Calvin Liebel]

Hello from the Pacific North West:
I seem to have inherited a problem from my previous boat. Water flowing back into the head from the holding tank. 
I use the KISS system of using the shower/sink fresh water as a source for flushing so usually leave the leaver in the Dry position all the time.
My problem is when I return to the head and find the bowl half full of (#*^@) I have tried leaving the selector leaver on the head in both directions (wet/dry) with no change in results. 
Asked a few owners about this problem, while some say "leave the leaver in the dry position", it would seem I am not the only one experiencing this problem.
The head works fine with only this hick-up, so have not tried to dismantle the unit to date.  Should I dive in and check the system or is there something I have missed in the instructions?  After all, I am a man and who needs to read manuals?
Cal Liebel 
Prairie Dream # 1056