I just read in this excellent article on joker valves for marine heads (http://www.practical-sailor.com/issues/37_31/features/Joker-Valves-for-Marine-Heads_11065-1.html#.VIzYJWD6HI0.twitter) that you should always install a duck-billed joker valve with the bill oriented vertically. If you install it horizontally -- one's natural inclination, given the name -- it will sag over time, and the valve will lose its seal. I realized this was probably why the forward head on my 1986 Carver Aft Cabin had a problem with the flushed waste-water seeping back into the bowl -- which, as you might imagine, leads to some rather unpleasant aromas. And staining, when the boat is left unused for a week or two.

I already had a PH II repair kit from Raritan, and my confidence was up from having successfully replaced the water intake valve on my aft head, so I decided to go ahead and tackle the project of replacing the joker valve on my forward head.

As luck would have it, the leaky valve had indeed been installed horizontally, and was probably several years old. The sagging was quite pronounced.

Lesson confirmed: install those duck-bills vertically.

And if your waste-water flows back into the bowl, replace the joker valve. It's not that hard. I'm a total newbie, and it took me about 45 minutes, including 10 minutes to pump out the holding tank before starting the job, to clear the effluent out of the drain hose.