I can't say for certain on your Groco. On the Raritan
there are two bolts that hold the flange that positions the joker valve. I had a noxious problem with the last bit in the waste hose slowly seeping back into the bowl. According to Peggy Hall, the "headmistress", if the bolts were overtightened they distorted the joker valve enough to let "water" seep back past the joker valve.
The last time I rebuilt the head I was extremely careful to alternately tighten each bolt as evenly as possible. Nor did I really torque them all that much. With many smiles,
, I can report no back seepage.
Short of a whole new head which may or may not solve the problem (probably not), I can only report my own small success. Of course I dread the next time it is time to re-build and can only hope I do it right again. At the very least, in the case of my Raritan
, I know what the problem is. According to Waldo in Robert Heinlein's novel, knowing the problem is 90% of the solution.